Finding A Year's Doomsday
We first take the anchor day for the century. For the purposes of the Doomsday rule, a century starts with '00 and ends with '99. The following table shows the anchor day of centuries 1800–1899, 1900–1999, 2000–2099 and 2100–2199.
| Century | Anchor day | Mnemonic | Index (day of week) |
|---|---|---|---|
| 1800–1899 | Friday | - | 5 (Fiveday) |
| 1900–1999 | Wednesday | We-in-dis-day (most living people were born in that century) |
3 (Treblesday) |
| 2000–2099 | Tuesday | Y-Tue-K or Twos-day (Y2K was at the head of this century) |
2 (Twosday) |
| 2100–2199 | Sunday | Twenty-one-day is Sunday (2100 is the start of the next century) |
0 (Noneday) |
Next, we find the year's Doomsday. To accomplish that according to Conway:
- Divide the year's last two digits (call this y) by 12 and let a be the floor of the quotient.
- Let b be the remainder of the same quotient.
- Divide that remainder by 4 and let c be the floor of the quotient.
- Let d be the sum of the three numbers (d = a + b + c). (It is again possible here to divide by seven and take the remainder. This number is equivalent, as it must be, to the sum of the last two digits of the year taken collectively plus the floor of those collective digits divided by four.)
- Count forward the specified number of days (d or the remainder of d/7) from the anchor day to get the year's Doomsday.
For the twentieth-century year 1966, for example:
As described in bullet 4, above, this is equivalent to:
So Doomsday in 1966 fell on Monday.
Similarly, Doomsday in 2005 is on a Monday:
Read more about this topic: Doomsday Rule
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