Key dates are a set of 10 dates that are spaced exactly 5 weeks apart (except week 1). These dates are special because their week number is the same for all years that aren't leap years.
| Week number | 1 | 5 | 10 | 15 | 20 | 25 | 30 | 35 | 40 | 45 | 50 |
|---|---|---|---|---|---|---|---|---|---|---|---|
| Key date | 4. January | 1. February | 8. March | 12. April | 17. May | 21. June | 26. July | 30. August | 4. October | 8. November | 13. December |
You'll need to memorize these.
However, I recommend only memorizing 3 dates, and deriving the others on the fly to begin with.
The ones to remember are: 1 (4. January), week 5 (1. February) and week 40 (4. October).
My mnemonic for week 40 is to turn the 0 into an O in my mind.
February has 28 days, so the key date for week 9 (5 + 4) is 1. March, easy!
It then follows that the key date for week 10 is 1 + 7 => 8. March.
The rule from here is to add 4 days and a month to get to the next key date if the month you're in has 31 days.
Add 5 days if it has 30.
Week 15: 8 + 4 => 12. April
Week 20: 12 + 5 => 17. May
Week 25: 17 + 4 => 21. June
Week 30: 21 + 5 => 26. July
Week 35: 26 + 4 => 30 August
Remember that week 40 is 4. October
Week 45: 4 + 4 => 8. November
Week 50: 8 + 5 => 13. December
We now have to figure out which weekday all the key dates land on. Just check which weekday January 4. lands on. In 2026, its on a Sunday. I therefore think of 2026 as a Sunday year.
If you need to check a year in the future or the past, then you can add or subtract 1 weekday per normal year, and 2 weekdays for a leapyear.
Going forward: 2027 (Monday year)
2028 (leap year, I'll cover these later, Tuesday/Wednesday)
2029 (Thursday year)
2030 (Friday year)
Going backwards: 2025 (Saturday year)
2024 (leap year Thursday/Friday)
2023 (Wednesday year)
2022 (Tuesday year)
If you need to go much further, use this formula to find the weekday of 1. January: \(year - 2000 + \lfloor\frac{year - 2000}{4}\rfloor \mod 7\)
This works because 1. January 2001 happens to be on a Monday And the rest is just a formula version of counting years and double-counting leap years.
Now just count the weekdays from 1. January, to 4. January, to get the key weekday.
Now all that remains is to find the closest key date to the target date,
and count by seven until you hit a date in the same week as the target.
Now you know the week number, and a date of a particular weekday in that week.
Count weekdays forward or backward until you hit the target date.
I'll cover leap years and examples later.