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01 · Foundation
Century anchors
Four numbers to remember: 1, 6, 4, 3. They give you the starting weekday for every century in a 400-year cycle.
How it works — step by step
Look at the start of the century: 1981 → 1900. Divide that century year by 400 and keep the remainder. Match 100, 200, 300 or 0 to 1, 6, 4 or 3.
| Century year | Remainder ÷ 400 | Anchor |
|---|---|---|
| 1700 | 100 | 1 · Sunday |
| 1800 | 200 | 6 · Friday |
| 1900 | 300 | 4 · Wednesday |
| 2000 | 0 | 3 · Tuesday |
1981 is in the 1900s. 1900 − 4 × 400 = 300. A remainder of 300 gives anchor 4: Wednesday. This is the century anchor, not yet the weekday for your date.
The cycle repeats: 2100 → 1, 2200 → 6, 2300 → 4, 2400 → 3. For years 1–99, use century year 0.
BDAY numbers the weekdays from Sunday = 1 to Saturday = 7. A remainder of 0 means Saturday when you turn a final weekday total into a day number.
Exercise
1994
This session only. Each exercise counts once.
Gregorian calendar, years 1–9999. Earlier dates are calculated using Gregorian rules extended backwards; historical local calendars may differ.