A quick question with a surprisingly exact answer — and an occasional twist. Here's the short version, the rule behind it, a reference table for every year from 2020 to 2040, and why the difference between a 52- and 53-week year quietly matters.
A calendar year has 52 weeks — almost always.
More precisely, 365 days is 52 weeks and 1 day, and a leap year adds a second spare day. Those leftover days accumulate, so under the ISO 8601 week system some years stretch to a 53rd week to keep weeks whole and aligned to Mondays.
The rule for 52 vs 53 weeks
The ISO 8601 week system has two rules: every week starts on Monday, and week 1 is the week containing the year's first Thursday (equivalently, the week with January 4th in it). Because weeks are whole Monday-to-Sunday blocks, a year occasionally needs a 53rd week to cover all its days. That happens in exactly two cases:
1 January is a Thursday — e.g. 2026 and 2037, or
1 January is a Wednesday and it's a leap year — e.g. 2020 and 2032.
Every other year has 52 weeks. That's roughly 71 years in every 400 that carry a 53rd week — about once every five or six years.
Weeks in each year, 2020–2040
The full count for every year in the range, with the dates of week 1. The ● rows are the 53-week years.
Year
ISO weeks
Week 1 (Mon – Sun)
2020
53
30 Dec 2019 – 5 Jan 2020
2021
52
4 Jan 2021 – 10 Jan 2021
2022
52
3 Jan 2022 – 9 Jan 2022
2023
52
2 Jan 2023 – 8 Jan 2023
2024
52
1 Jan 2024 – 7 Jan 2024
2025
52
30 Dec 2024 – 5 Jan 2025
2026
53
29 Dec 2025 – 4 Jan 2026
2027
52
4 Jan 2027 – 10 Jan 2027
2028
52
3 Jan 2028 – 9 Jan 2028
2029
52
1 Jan 2029 – 7 Jan 2029
2030
52
31 Dec 2029 – 6 Jan 2030
2031
52
30 Dec 2030 – 5 Jan 2031
2032
53
29 Dec 2031 – 4 Jan 2032
2033
52
3 Jan 2033 – 9 Jan 2033
2034
52
2 Jan 2034 – 8 Jan 2034
2035
52
1 Jan 2035 – 7 Jan 2035
2036
52
31 Dec 2035 – 6 Jan 2036
2037
53
29 Dec 2036 – 4 Jan 2037
2038
52
4 Jan 2038 – 10 Jan 2038
2039
52
3 Jan 2039 – 9 Jan 2039
2040
52
2 Jan 2040 – 8 Jan 2040
In this range the 53-week years are 2020, 2026, 2032, 2037. Note that week 1 often begins in late December of the previous year — that's normal, because the first Monday-to-Sunday week of the year can start before 1 January.
Why the 53rd week matters
For most people the difference is invisible. For some it's a recurring headache:
Payroll. Organisations that pay weekly or fortnightly occasionally hit a year with an extra pay period — a "53-week year" or "27th fortnight" — which has to be handled deliberately so annual salaries and tax come out right.
Retail & finance. Companies on a 4-4-5 or 13-week-quarter calendar add an extra week roughly every five to six years to resync with the real calendar; that "53-week year" makes year-on-year comparisons tricky.
Planning & scheduling. Anyone running a project or rota by week number needs to know whether they're working with 52 or 53 slots before the year turns.
How many working weeks?
If you mean weeks spent at work rather than weeks on the calendar, the figure is smaller. Subtract annual leave and public holidays — typically four to six weeks in much of Europe — and a 52-week year usually leaves around 46–48 working weeks. The exact number depends on your contract and country, so it's best counted directly.