FIELD GUIDE / BOUNDARY LAB
The year ends. The week doesn’t.
December 31, 2026 is Thursday. January 1, 2027 is Friday. A civil-year boundary changes the year label, not the uninterrupted seven-day cycle.
01 / The crossing row
In a Monday-first view, the shared boundary week is December 28, 29, 30, 31 followed by January 1, 2, 3. Both month views show the same seven dates; only the emphasis changes.
02 / Context on either side
November 2026 begins Sunday and ends Monday. December begins Tuesday and ends Thursday. January 2027 begins Friday and ends Sunday. February begins Monday and ends Sunday. The four specimens expose the transitions without resetting weekday order.
03 / Week-year is not calendar-year
Under ISO week numbering, January 1–3, 2027 belong to week 53 of week-year 2026. ISO week 1 of 2027 begins Monday, January 4. Do not confuse a date’s calendar year with its week-numbering year.