Weeks are numbered 1 to 52 — or 53 in long years — following the ISO 8601 standard: week 1 is the one containing the first Thursday of January, and every week runs Monday to Sunday.

Worth knowing: a week belongs to the year of its Thursday. In late December the number can therefore switch to week 1 of the following year before the 31st. This is the number that calendars, team rosters and shared-custody rhythms rely on.