The 53 weeks of 2027, each with the dates it runs from and to.
Weeks start on Sunday, and week 1 is the one containing 1 January (US convention).
| Week | Dates |
|---|---|
| 1 | Dec 27 – Jan 2 |
| 2 | Jan 3 – Jan 9 |
| 3 | Jan 10 – Jan 16 |
| 4 | Jan 17 – Jan 23 |
| 5 | Jan 24 – Jan 30 |
| 6 | Jan 31 – Feb 6 |
| 7 | Feb 7 – Feb 13 |
| 8 | Feb 14 – Feb 20 |
| 9 | Feb 21 – Feb 27 |
| 10 | Feb 28 – Mar 6 |
| 11 | Mar 7 – Mar 13 |
| 12 | Mar 14 – Mar 20 |
| 13 | Mar 21 – Mar 27 |
| 14 | Mar 28 – Apr 3 |
| 15 | Apr 4 – Apr 10 |
| 16 | Apr 11 – Apr 17 |
| 17 | Apr 18 – Apr 24 |
| 18 | Apr 25 – May 1 |
| 19 | May 2 – May 8 |
| 20 | May 9 – May 15 |
| 21 | May 16 – May 22 |
| 22 | May 23 – May 29 |
| 23 | May 30 – Jun 5 |
| 24 | Jun 6 – Jun 12 |
| 25 | Jun 13 – Jun 19 |
| 26 | Jun 20 – Jun 26 |
| 27 | Jun 27 – Jul 3 |
| 28 | Jul 4 – Jul 10 |
| 29 | Jul 11 – Jul 17 |
| 30 | Jul 18 – Jul 24 |
| 31 | Jul 25 – Jul 31 |
| 32 | Aug 1 – Aug 7 |
| 33 | Aug 8 – Aug 14 |
| 34 | Aug 15 – Aug 21 |
| 35 | Aug 22 – Aug 28 |
| 36 | Aug 29 – Sep 4 |
| 37 | Sep 5 – Sep 11 |
| 38 | Sep 12 – Sep 18 |
| 39 | Sep 19 – Sep 25 |
| 40 | Sep 26 – Oct 2 |
| 41 | Oct 3 – Oct 9 |
| 42 | Oct 10 – Oct 16 |
| 43 | Oct 17 – Oct 23 |
| 44 | Oct 24 – Oct 30 |
| 45 | Oct 31 – Nov 6 |
| 46 | Nov 7 – Nov 13 |
| 47 | Nov 14 – Nov 20 |
| 48 | Nov 21 – Nov 27 |
| 49 | Nov 28 – Dec 4 |
| 50 | Dec 5 – Dec 11 |
| 51 | Dec 12 – Dec 18 |
| 52 | Dec 19 – Dec 25 |
| 53 | Dec 26 – Jan 1 |
Weeks here start on Sunday and week 1 is the one containing 1 January, which is the convention used in the United States. Most of Europe uses ISO-8601 instead: weeks run Monday to Sunday, and week 1 is the one containing the first Thursday of the year. The two give different numbers for much of the year, so a week number is only unambiguous once you know which system produced it.
Under ISO-8601 the year boundary does not align with the week boundary. When 1 January falls on a Friday, Saturday or Sunday, those days belong to the last week of the previous year — which is why a week number always travels with a year, and why «week 53» on its own is ambiguous.
Week numbers are administrative rather than astronomical. They appear in contracts, delivery schedules, shift rotas and production planning, and they exist so that two parties can name a stretch of seven days without spelling out both dates.
2027 has 52 weeks, which is the ordinary case.
The US convention: weeks start on Sunday, and week 1 is the one containing 1 January. The Portuguese, Spanish, German and French versions of this page use ISO-8601 instead, because that is the standard in those markets.
Because 52 weeks are 364 days, one short of a year. The extra day accumulates, and roughly every five or six years it adds up to a whole extra week. Under ISO-8601 it happens when the year begins on a Thursday, or on a Wednesday in a leap year.
A week belongs to exactly one year, but not always the one its days suggest. Under ISO-8601, 1 January 2027 falls on a Friday and belongs to week 53 of 2026 — the first three days of the calendar year are part of the previous week-year.
They are the same thing under different names. German uses Kalenderwoche, abbreviated KW, and it appears routinely in contracts and delivery dates. English speakers say week number, and use it far less often in everyday business.
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