A week starts on Monday, not on Sunday: https://en.wikipedia.org/wiki/ISO_week_date "An ISO week-numbering year (also called ISO year informally) has 52 or 53 full weeks. That is 364 or 371 days instead of the usual 365 or 366 days. The extra week is sometimes referred to as a leap week, although ISO 8601 does not use this term.
Weeks start with Monday and end on Sunday. ..."
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