Week Numbers for 2021 — ISO Week List

Week numbers for 2021

2021 has 52 ISO weeks. Week 1 starts on Jan 04, 2021 and week 52 ends on Jan 02, 2022. Each week below shows its ISO week number, the Monday start date, the Sunday end date, and the Unix epoch timestamp for the start of the week (00:00 UTC on Monday). Weeks are numbered per ISO 8601, where week 1 is the week containing the first Thursday of the year and every week runs Monday to Sunday.

Epoch columns are UTC: "Start" is 00:00:00 on the Monday, "End" is 00:00:00 on the Sunday — the first second of the last day, not the last second of the week. For week 1 of 2021, that end value is 1610236800; add 604,799 to any start epoch to land on the week's final second (Sunday 23:59:59).

How to read this table

• Week # — the ISO 8601 week number for that week.
• ISO year — the ISO week-numbering year this week belongs to (can differ from the calendar year for a few edge days).
• Monday start — the date of the Monday that starts the week, in UTC.
• Sunday end — the date of the Sunday that ends the week, in UTC.
• Start epoch (UTC) — Unix timestamp for 00:00 UTC on Monday.
• End epoch (UTC) — Unix timestamp for 00:00 UTC on Sunday.

Reading 2021's weeks as epoch ranges

A Unix timestamp pins an instant; an ISO week names a seven-day span. Reporting tools constantly translate between the two — a log line becomes "week 9", or a weekly report becomes an epoch range to query. In 2021, that mapping is concrete: week 1 opens at epoch 1609718400, and week 52 closes at 1641686399, the final second of Sunday, Jan 02, 2022.

• Epoch 1609718400 → 00:00 UTC on Monday of week 1 (Jan 04, 2021).
• Epoch 1641686399 → 23:59:59 UTC on Sunday of week 52 — one second before 1641686400 would tick over to the next ISO year's week 1.
• Any week's span → its start epoch up to start + 604,799 seconds; exactly 604,800 seconds per week, no exceptions (UTC has no DST).

Why 2021 has 52 weeks

The count falls out of one rule: ISO week 1 is the week containing the first Thursday, and weeks run Monday–Sunday. 2021 starts on a Friday, which keeps it a 52-week year; the days after week 52 belong to week 1 of 2022. December 31, $2021 closes on a $Friday, and the last week ends $in January 2022

How 2021's weeks line up against neighboring years

The same calendar date rarely keeps its week number from one year to the next, because each January 1 lands on a different weekday. March 1, 2021 sits in ISO week 9; in 2020 and 2022 it will sit one week off in either direction. The tables for the adjacent years make the drift visible:

• Week numbers for 2024 (52 weeks)
• Week numbers for 2025 (52 weeks)
• Week numbers for 2026 (53 weeks)
• Week numbers for 2020
• Week numbers for 2022

Examples: 2021's weeks at the epoch boundaries

• January 1, 2021 (Friday) falls in ISO week 53 of ISO year 2020 — the ISO year differs from the calendar year, exactly as the rule predicts.
• December 31, 2021 (Friday) falls in ISO week 52 of ISO year 2021.
• March 1, 2021 sits in ISO week 9 — the sort of date whose week number shifts year to year.

Week Numbers for 2021 — ISO Week List — questions from actual use

What are the week numbers for 2021?

2021 has 52 ISO weeks. Week 1 runs Jan 04, 2021 to Jan 10, 2021, and week 52 runs Dec 27, 2021 to Jan 02, 2022. The final week spills into January 2022.

Is 2021 a 52-week or 53-week year?

2021 is a 52-week year (January 1 was a Friday). The next 53-week year is 2026, because its January 1 falls on the right weekday to spill into a 53rd week.

When do 2021's week numbers and dates disagree?

Day-of-year and week-of-year counting disagree at the edges. January 1, 2021 (Friday) sits in ISO week 1 of 2021, and December 31 (Friday) lands in week 52 of 2021. The ISO year owns the days its weeks' Thursdays fall in, so a few days each December and January always belong to the neighboring year.

What epoch timestamps belong to a week in 2021?

Each week on this page shows two Unix timestamps: 00:00 UTC on its Monday (the start) and 00:00 UTC on its Sunday. For 2021, those run from 1609718400 for the start of week 1 to 1641686399 for the last second of week 52 — a span of exactly 31,449,600 seconds across 52 weeks.