019.py

def solution():
    days_per_month = [31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31]

    day = 6
    month = 1
    year = 1901

    sundays = 0

    while year < 2001:
        day += 7

        if (year % 4 == 0 and year % 100 != 0) or (year % 400 == 0):
            if day > days_per_month[month - 1] and month != 2:
                month += 1
                day = day - days_per_month[month - 2]
            elif day > 29 and month == 2:
                month += 1
                day = day - 29
        else:
            if day > days_per_month[month - 1]:
                month += 1
                day = day - days_per_month[month - 2]

        if month > 12:
            year += 1
            month = 1

        if year < 2001 and day == 1:
            sundays += 1
    return sundays

print(solution())