A year has 52 or 53 ISO weeks. Around New Year, dates can belong to the previous or next ISO year: December 29, 2025 is in week 1 of 2026.
Weekly leaderboards often use ISO weeks so every week is exactly seven days and starts on the same weekday.
Why it matters when you launch
FinderLaunch's weekly leaderboards group launches by ISO week, Monday to Sunday UTC.