diff --git a/jollyday-core/src/main/java/de/focus_shift/jollyday/core/spi/ChristianHoliday.java b/jollyday-core/src/main/java/de/focus_shift/jollyday/core/spi/ChristianHoliday.java
index c2705224a..b6a2404f1 100644
--- a/jollyday-core/src/main/java/de/focus_shift/jollyday/core/spi/ChristianHoliday.java
+++ b/jollyday-core/src/main/java/de/focus_shift/jollyday/core/spi/ChristianHoliday.java
@@ -2,8 +2,37 @@
 
 import java.time.chrono.Chronology;
 
+/**
+ * Represents a christian holiday that can be
+ *
+ * <ul>
+ *   <li>Movable</li>
+ *   <li>Limited</li>
+ *   <li>Described</li>
+ * </ul>
+ *
+ * and has a special {@link ChristianHolidayType} and a chronology based on {@link Chronology}.
+ * Two chronologies are supported:
+ * <ul>
+ *   <li>ISO</li>
+ *   <li>Julian</li>
+ * </ul>
+ *
+ * The {@link de.focus_shift.jollyday.core.parser.impl.ChristianHolidayParser} is used.
+ */
 public interface ChristianHoliday extends Limited, Described, Movable {
+
+  /**
+   * Describes the {@link ChristianHolidayType}
+   *
+   * @return the type of the christian holiday
+   */
   ChristianHolidayType type();
 
+  /**
+   * Describes the {@link Chronology} of the christian holiday
+   *
+   * @return the chronology of the christian holiday
+   */
   Chronology chronology();
 }
diff --git a/jollyday-core/src/main/java/de/focus_shift/jollyday/core/spi/Configuration.java b/jollyday-core/src/main/java/de/focus_shift/jollyday/core/spi/Configuration.java
index fba908d66..3a1d024fa 100644
--- a/jollyday-core/src/main/java/de/focus_shift/jollyday/core/spi/Configuration.java
+++ b/jollyday-core/src/main/java/de/focus_shift/jollyday/core/spi/Configuration.java
@@ -3,16 +3,45 @@
 import java.util.stream.Stream;
 
 /**
- * Represents the configuration.
+ * Represents the holiday configuration, meta information like the hierarchy and description
+ * and the sub configuration for a specific {@link de.focus_shift.jollyday.core.HolidayCalendar}.
+ * <ul>
+ *   <li>The `holidays` contains the holiday configuration for every holiday.</li>
+ *   <li>The `hierarchy` contains the ISO 3166-1 alpha-2 for countries or ISO 3166-2 code for subdivisions.</li>
+ *   <li>The `description` contains further information.</li>
+ *   <li>The `subConfiguration` contains the holiday configuration for subdivisions, cities, ... of the main configuration.</li>
+ * </ul>
  */
 public interface Configuration {
 
-  Holidays holidays();
-
-  Stream<Configuration> subConfigurations();
-
+  /**
+   * Contains the ISO 3166-1 alpha-2 for countries or ISO 3166-2 code for subdivisions
+   *
+   * @return the code of this configuration
+   */
   String hierarchy();
 
+  /**
+   * Contains further information like the name.
+   *
+   * @return the description of this configuration
+   */
   String description();
 
+  /**
+   * Contains all holiday configuration of different types like {@link Fixed} or {@link ChristianHoliday} e.g.
+   *
+   * @return all holiday configuration for this {@link de.focus_shift.jollyday.core.HolidayCalendar}
+   */
+  Holidays holidays();
+
+  /**
+   * Contains the holiday configuration for subdivisions, cities, ... of the main configuration.
+   * The hierarchy is built on top of the iso standard. For germany e.g. there is a main configuration for Germany
+   * and 16 sub configurations for the subdivisions.
+   *
+   * @return all configurations for the next hierarchy level like subdivisions or cities.
+   */
+  Stream<Configuration> subConfigurations();
+
 }
diff --git a/jollyday-core/src/main/java/de/focus_shift/jollyday/core/spi/ConfigurationService.java b/jollyday-core/src/main/java/de/focus_shift/jollyday/core/spi/ConfigurationService.java
index cb43f482b..1fdbf03bb 100644
--- a/jollyday-core/src/main/java/de/focus_shift/jollyday/core/spi/ConfigurationService.java
+++ b/jollyday-core/src/main/java/de/focus_shift/jollyday/core/spi/ConfigurationService.java
@@ -8,4 +8,5 @@
 public interface ConfigurationService {
 
   Configuration getConfiguration(ManagerParameter parameter);
+
 }
diff --git a/jollyday-core/src/main/java/de/focus_shift/jollyday/core/spi/Described.java b/jollyday-core/src/main/java/de/focus_shift/jollyday/core/spi/Described.java
index 635de5a2a..75e1c553f 100644
--- a/jollyday-core/src/main/java/de/focus_shift/jollyday/core/spi/Described.java
+++ b/jollyday-core/src/main/java/de/focus_shift/jollyday/core/spi/Described.java
@@ -2,8 +2,25 @@
 
 import de.focus_shift.jollyday.core.HolidayType;
 
+/**
+ * Provides the functionality to describe a holiday with a
+ * name represented by the `descriptionPropertiesKey` and a
+ * `officiality` holiday type that can be one of the provides {@link HolidayType}s
+ */
 public interface Described {
+
+  /**
+   * The properties key will be used to translate the holiday into the requested language
+   *
+   * @return the properties key to use to translate
+   */
   String descriptionPropertiesKey();
 
+  /**
+   * Represents the type of the holiday.
+   * Different holiday types are available in {@link HolidayType}
+   *
+   * @return the holiday type
+   */
   HolidayType officiality();
 }
diff --git a/jollyday-core/src/main/java/de/focus_shift/jollyday/core/spi/EthiopianOrthodoxHoliday.java b/jollyday-core/src/main/java/de/focus_shift/jollyday/core/spi/EthiopianOrthodoxHoliday.java
index 172510004..92bb24d70 100644
--- a/jollyday-core/src/main/java/de/focus_shift/jollyday/core/spi/EthiopianOrthodoxHoliday.java
+++ b/jollyday-core/src/main/java/de/focus_shift/jollyday/core/spi/EthiopianOrthodoxHoliday.java
@@ -1,5 +1,24 @@
 package de.focus_shift.jollyday.core.spi;
 
+/**
+ * Represents an ethiopian orthodox holiday that can be
+ *
+ * <ul>
+ *   <li>Limited</li>
+ *   <li>Described</li>
+ * </ul>
+ *
+ * and has a special {@link EthiopianOrthodoxHolidayType}.
+ * The {@link de.focus_shift.jollyday.core.parser.impl.EthiopianOrthodoxHolidayParser} is used.
+ *
+ */
 public interface EthiopianOrthodoxHoliday extends Described, Limited {
+
+  /**
+   * Describes the {@link EthiopianOrthodoxHolidayType}
+   *
+   * @return the type of the ethiopian orthodox holiday
+   */
   EthiopianOrthodoxHolidayType type();
+
 }
diff --git a/jollyday-core/src/main/java/de/focus_shift/jollyday/core/spi/Fixed.java b/jollyday-core/src/main/java/de/focus_shift/jollyday/core/spi/Fixed.java
index 008ed99f6..32e2118bb 100644
--- a/jollyday-core/src/main/java/de/focus_shift/jollyday/core/spi/Fixed.java
+++ b/jollyday-core/src/main/java/de/focus_shift/jollyday/core/spi/Fixed.java
@@ -3,5 +3,7 @@
 import java.time.MonthDay;
 
 public interface Fixed extends Described, Movable, Limited {
+
   MonthDay day();
+
 }
diff --git a/jollyday-core/src/main/java/de/focus_shift/jollyday/core/spi/FixedWeekdayBetweenFixed.java b/jollyday-core/src/main/java/de/focus_shift/jollyday/core/spi/FixedWeekdayBetweenFixed.java
index 374e1110a..0a27bca4d 100644
--- a/jollyday-core/src/main/java/de/focus_shift/jollyday/core/spi/FixedWeekdayBetweenFixed.java
+++ b/jollyday-core/src/main/java/de/focus_shift/jollyday/core/spi/FixedWeekdayBetweenFixed.java
@@ -3,9 +3,11 @@
 import java.time.DayOfWeek;
 
 public interface FixedWeekdayBetweenFixed extends Described, Limited {
+
   Fixed from();
 
   Fixed to();
 
   DayOfWeek weekday();
+
 }
diff --git a/jollyday-core/src/main/java/de/focus_shift/jollyday/core/spi/FixedWeekdayInMonth.java b/jollyday-core/src/main/java/de/focus_shift/jollyday/core/spi/FixedWeekdayInMonth.java
index b1c7da63f..23086af5b 100644
--- a/jollyday-core/src/main/java/de/focus_shift/jollyday/core/spi/FixedWeekdayInMonth.java
+++ b/jollyday-core/src/main/java/de/focus_shift/jollyday/core/spi/FixedWeekdayInMonth.java
@@ -4,9 +4,11 @@
 import java.time.Month;
 
 public interface FixedWeekdayInMonth extends Described, Limited {
+
   DayOfWeek weekday();
 
   Month month();
 
   Occurrance which();
+
 }
diff --git a/jollyday-core/src/main/java/de/focus_shift/jollyday/core/spi/FixedWeekdayRelativeToFixed.java b/jollyday-core/src/main/java/de/focus_shift/jollyday/core/spi/FixedWeekdayRelativeToFixed.java
index 64ebe557a..aabed41bc 100644
--- a/jollyday-core/src/main/java/de/focus_shift/jollyday/core/spi/FixedWeekdayRelativeToFixed.java
+++ b/jollyday-core/src/main/java/de/focus_shift/jollyday/core/spi/FixedWeekdayRelativeToFixed.java
@@ -3,6 +3,7 @@
 import java.time.DayOfWeek;
 
 public interface FixedWeekdayRelativeToFixed extends Described, Limited {
+
   Fixed day();
 
   Occurrance which();
@@ -10,4 +11,5 @@ public interface FixedWeekdayRelativeToFixed extends Described, Limited {
   DayOfWeek weekday();
 
   Relation when();
+
 }
diff --git a/jollyday-core/src/main/java/de/focus_shift/jollyday/core/spi/IslamicHoliday.java b/jollyday-core/src/main/java/de/focus_shift/jollyday/core/spi/IslamicHoliday.java
index 1d4b1485b..a4e538d63 100644
--- a/jollyday-core/src/main/java/de/focus_shift/jollyday/core/spi/IslamicHoliday.java
+++ b/jollyday-core/src/main/java/de/focus_shift/jollyday/core/spi/IslamicHoliday.java
@@ -1,5 +1,19 @@
 package de.focus_shift.jollyday.core.spi;
 
+/**
+ * Represents a islamic holiday that can be
+ *
+ * <ul>
+ *   <li>Movable</li>
+ *   <li>Limited</li>
+ *   <li>Described</li>
+ * </ul>
+ *
+ * and has a special {@link IslamicHolidayType}.
+ * The {@link de.focus_shift.jollyday.core.parser.impl.IslamicHolidayParser} is used.
+ */
 public interface IslamicHoliday extends Described, Limited, Movable {
+
   IslamicHolidayType type();
+
 }
diff --git a/jollyday-core/src/main/java/de/focus_shift/jollyday/core/spi/Limited.java b/jollyday-core/src/main/java/de/focus_shift/jollyday/core/spi/Limited.java
index bacade4a9..0ac9ce8a8 100644
--- a/jollyday-core/src/main/java/de/focus_shift/jollyday/core/spi/Limited.java
+++ b/jollyday-core/src/main/java/de/focus_shift/jollyday/core/spi/Limited.java
@@ -2,10 +2,44 @@
 
 import java.time.Year;
 
+/**
+ * Provides the functionality to limit the years a holiday will occur.
+ * By default, a holiday will occur on every year it is requested until
+ * this interface will be used and valid from or valid to is set, then the
+ * holiday will only occur in this year's. If the cycle is set, a holiday will
+ * only occur in the years represented by the cycle.
+ * <p>
+ * Example:
+ * if a holiday is only valid from 1900 until 1910 and a cycle from five years
+ * is configured, then the holiday will occur in the years:
+ * <ul>
+ *   <li>1900</li>
+ *   <li>1905</li>
+ *   <li>1910</li>
+ * </ul>
+ */
 public interface Limited {
+
+  /**
+   * Describes the first year a holiday is valid (inclusive)
+   *
+   * @return the first valid year of a holiday
+   */
   Year validFrom();
 
+  /**
+   * Describes the last year a holiday is valid (inclusive)
+   *
+   * @return the last valid year of a holiday
+   */
   Year validTo();
 
+  /**
+   * Describes the cycle a holiday is valid in between the
+   * years given bei from and to.
+   *
+   * @return the cycle to limit a holiday between from and to
+   */
   YearCycle cycle();
+
 }
diff --git a/jollyday-core/src/main/java/de/focus_shift/jollyday/core/spi/Movable.java b/jollyday-core/src/main/java/de/focus_shift/jollyday/core/spi/Movable.java
index 0e68b0155..22868fe04 100644
--- a/jollyday-core/src/main/java/de/focus_shift/jollyday/core/spi/Movable.java
+++ b/jollyday-core/src/main/java/de/focus_shift/jollyday/core/spi/Movable.java
@@ -3,5 +3,13 @@
 import java.util.List;
 
 public interface Movable {
+
+  /**
+   * Describes the different moving conditions that
+   * have been configured for a specific holiday.
+   *
+   * @return the configured {@link MovingCondition} for this holiday
+   */
   List<MovingCondition> conditions();
+
 }
diff --git a/jollyday-core/src/main/java/de/focus_shift/jollyday/core/spi/MovingCondition.java b/jollyday-core/src/main/java/de/focus_shift/jollyday/core/spi/MovingCondition.java
index c1ac6eac0..de5e78a8f 100644
--- a/jollyday-core/src/main/java/de/focus_shift/jollyday/core/spi/MovingCondition.java
+++ b/jollyday-core/src/main/java/de/focus_shift/jollyday/core/spi/MovingCondition.java
@@ -3,9 +3,11 @@
 import java.time.DayOfWeek;
 
 public interface MovingCondition {
+
   DayOfWeek substitute();
 
   With with();
 
   DayOfWeek weekday();
+
 }
diff --git a/jollyday-core/src/main/java/de/focus_shift/jollyday/core/spi/RelativeToEasterSunday.java b/jollyday-core/src/main/java/de/focus_shift/jollyday/core/spi/RelativeToEasterSunday.java
index 66f6d31d7..813e04efa 100644
--- a/jollyday-core/src/main/java/de/focus_shift/jollyday/core/spi/RelativeToEasterSunday.java
+++ b/jollyday-core/src/main/java/de/focus_shift/jollyday/core/spi/RelativeToEasterSunday.java
@@ -5,7 +5,9 @@
 import java.time.chrono.Chronology;
 
 public interface RelativeToEasterSunday extends Described, Limited {
+
   Chronology chronology();
 
   Days days();
+
 }
diff --git a/jollyday-core/src/main/java/de/focus_shift/jollyday/core/spi/RelativeToFixed.java b/jollyday-core/src/main/java/de/focus_shift/jollyday/core/spi/RelativeToFixed.java
index 70d459c88..e7adfa40f 100644
--- a/jollyday-core/src/main/java/de/focus_shift/jollyday/core/spi/RelativeToFixed.java
+++ b/jollyday-core/src/main/java/de/focus_shift/jollyday/core/spi/RelativeToFixed.java
@@ -5,6 +5,7 @@
 import java.time.DayOfWeek;
 
 public interface RelativeToFixed extends Described, Limited {
+
   Fixed date();
 
   DayOfWeek weekday();
@@ -12,4 +13,5 @@ public interface RelativeToFixed extends Described, Limited {
   Relation when();
 
   Days days();
+
 }
diff --git a/jollyday-core/src/main/java/de/focus_shift/jollyday/core/spi/RelativeToWeekdayInMonth.java b/jollyday-core/src/main/java/de/focus_shift/jollyday/core/spi/RelativeToWeekdayInMonth.java
index 4b4c384ab..0f31ac70d 100644
--- a/jollyday-core/src/main/java/de/focus_shift/jollyday/core/spi/RelativeToWeekdayInMonth.java
+++ b/jollyday-core/src/main/java/de/focus_shift/jollyday/core/spi/RelativeToWeekdayInMonth.java
@@ -3,9 +3,11 @@
 import java.time.DayOfWeek;
 
 public interface RelativeToWeekdayInMonth extends Described, Limited {
+
   FixedWeekdayInMonth weekdayInMonth();
 
   DayOfWeek weekday();
 
   Relation when();
+
 }
diff --git a/jollyday-tests/src/test/java/de/focus_shift/jollyday/tests/CalendarCheckerApi.java b/jollyday-tests/src/test/java/de/focus_shift/jollyday/tests/CalendarCheckerApi.java
index ab90d254a..9086800e6 100644
--- a/jollyday-tests/src/test/java/de/focus_shift/jollyday/tests/CalendarCheckerApi.java
+++ b/jollyday-tests/src/test/java/de/focus_shift/jollyday/tests/CalendarCheckerApi.java
@@ -15,7 +15,7 @@ public interface CalendarCheckerApi {
    * <p>
    * Calling multiple methods on the returned {@link CalendarChecker} is safe as it only interacts with the {@link CalendarCheckerApi}
    * <p>
-   * Examples:
+   * Example:
    * <pre><code class='java'> // you can chain multiple holiday checks
    * assertFor(GERMANY)
    *   .hasFixedHoliday("NEW_YEAR", JANUARY, 1).and()