Short answer
First and last week of a year
Determine the start of week 1 and the end of the final ISO week.
Short answer
The first week contains 4 January; the final week contains 28 December.
How it works
Either range may cross a calendar-year boundary.
Find the first Monday and last Sunday
Find 4 January and move back to the Monday of its week: that starts week 1. Find 28 December to identify the final ISO week; its Sunday may fall in the next calendar year.
2026-W01 is 29 December 2025–4 January 2026. 2026-W53 is 28 December 2026–3 January 2027. Week 1 of 2027 starts on 4 January. A January month view can therefore legitimately begin with week 52 or 53 of the preceding ISO year.