What Week: Week 1
Find the calendar dates covered by any ISO week number.
ISO Week 1 of 2026 runs from December 29 to January 4, 2026, spanning two calendar months, with roughly 51 ISO weeks remaining in the year after it. ISO 8601 weeks always run Monday through Sunday and belong to the year containing that week's Thursday, which is why the first few days of January sometimes belong to the final week of the previous year. New Year's Day falls within this week in 2026. Next year, 2027, the same week number runs from January 4 to January 10, 2027 instead, since the exact dates shift slightly every year.
As of October 1, 2026, 12:19 PM (America/New York).
Position in the Year
Week 1 falls in the first half of the year, roughly 2% of the way through 2026's 52-53 ISO weeks.
Quarter Overlap
This week sits in calendar Q4, based on its Monday (December 29).