Week number
What week of the year is it? Here's today's ISO 8601 week number, or the week number of any date you want to check.
The ISO 8601 rule: Thursday decides
Under the ISO 8601 standard, weeks start on Monday and week 1 is the week that contains the first Thursday of the year (equivalently, the week that contains January 4). That's why January 1 or 2 can still belong to the last week of the previous year — week 52 or 53 — and late December can already count as "week 1" of the new year. Each week belongs to whichever year its Thursday falls in. Note that this differs from the US convention used by some calendars, where weeks start on Sunday and week 1 simply contains January 1.
52 or 53 weeks? And what week numbers are for
Almost every year has 52 ISO weeks; a year has 53 when it starts on a Thursday, or when it's a leap year starting on a Wednesday (it happened in 2020 and happens again in 2026 and 2032). Week numbers are everywhere in business — the "CW" (calendar week) you see in supplier emails, shipping schedules, and manufacturing plans — and they're also used in schools and hospitals to organize rotations and shifts.