What Week of the Year Is It?

How do ISO week numbers work?

ISO 8601 weeks always start on Monday. Week 1 is the week containing the year's first Thursday, which is the same as the week containing January 4. A year has either 52 or 53 ISO weeks.

Why does the ISO week sometimes differ from "week of the year"?

Some calendars count week 1 as the week containing January 1, regardless of weekday. ISO 8601 only counts a week as belonging to the year if it contains at least four days of that year. That's why early January can sometimes show as week 52 or 53 of the previous year.