Not every “week number” is calculated the same way, and the ISO-8601 standard used here has two rules that trip people up if they’re used to a more casual system. First, ISO weeks start on Monday, not Sunday — so a week runs Monday through Sunday, unlike the Sunday-through-Saturday layout common on many US calendars and some spreadsheet defaults. Second, week 1 of a year isn’t simply “the week containing January 1st.” It’s defined as the week that contains the year’s first Thursday.
That second rule has a genuinely surprising consequence at the edges of the year. Because week 1 is anchored to the first Thursday rather than to January 1st itself, the first few days of January can actually belong to week 52 or week 53 of the previous year if January 1st falls late in its week (say, a Friday, Saturday, or Sunday) — the new year starts mid-week-52, not mid-week-1. The same logic works in reverse at the other end: the last few days of December can already belong to week 1 of the next year if December 31st happens to fall early in its week. So it’s entirely possible to be looking at a date in late December and have it reported as week 1 of the following year, or a date in early January reported as a week from the year before.
This standard is deliberately unambiguous, which is exactly why it’s the default for international business systems, ISO-compliant software, and scheduling tools that need a week number nobody has to argue about. The number shown here is calculated fresh from your device’s local date each time the page loads.