What week of the year is it?
The ISO 8601 week number for any date, with the week-numbering year.
The part that catches people out
ISO weeks run Monday to Sunday, and week 1 is the week containing the first Thursday of January. That means the first days of a year can belong to week 52 or 53 of the year before — 1 January 2027 is in week 53 of 2026.