Two different systems share the same name
'Week number' means two different things depending on who's asking. The ISO 8601 international standard — used by most of Europe, project management tools, and software systems — starts weeks on Monday and defines Week 1 as the week containing the year's first Thursday (equivalently, the week containing January 4th). The US convention most American calendars and spreadsheets default to starts weeks on Sunday and simply counts Week 1 as the week containing January 1st, no matter which weekday that falls on. The two systems can disagree on the week number for the same date by exactly 1, and disagree on which year a late-December or early-January date belongs to — which is the source of most 'why does my spreadsheet show a different week number' confusion.