ISO Week Numbers
ISO 8601 weeks start on Monday, and week 1 is the week containing the first Thursday of the year — which means January 1st can belong to week 52 or 53 of the previous year, and December 29–31 can belong to week 1 of the next. This page gets those edges right.
Date → week
Week → dates
Years with 53 weeks
A year has 53 ISO weeks when it starts on a Thursday, or is a leap year starting on a Wednesday.
Not the only week numbering! The US convention (and Excel’s default
WEEKNUM)
counts the week containing January 1 as week 1 and starts weeks on Sunday, so its numbers can differ from
ISO by one. If two systems disagree about a week number, this is almost always why.