Default first day of week should be Monday, per ISO 8601

[koying]

Touchy subject, but ISO 8601 univocally define Monday as the first day of week, so that should be the default.
At least that way, you have an objective reason to choose one day amongst Saturday/Sunday/Monday.

[phpdev-expert]

its have option like which day you want
when you provide :locale-data props its have option of firstDay, if firstDay=0 mean week will start from sunday and if firstDay = 1 mean week will start from modnay