DOOMSDAY Logs

As numbers 0–6, starting from Sunday — so all the arithmetic is done mod 7. Day # Sunday 0 Monday 1 Tuesday 2 Wednesday 3 Thursday 4 Friday 5 Saturday 6 Every step of Doomsday adds or subtracts small numbers and reduces mod 7 (keep the remainder after dividi...

Quiz: DOOMSDAY / What Is Doomsday | updated: Jul 14, 2026

A doomsday for each century's "00" year: 1800s → Friday, 1900s → Wednesday, 2000s → Tuesday, 2100s → Sunday. Century Anchor # 1700s / 2100s Sunday 0 1800s / 2200s Friday 5 1900s / 2300s Wednesday 3 2000s / 1600s Tuesday 2 The pattern repeats every 400 years (the G...

Quiz: DOOMSDAY / Year & Century Anchors | updated: Jul 14, 2026

Even months double up; odd months follow "9-to-5 at 7-11"; Jan/Feb shift for leap years; March uses Pi Day. Month Doomsday date Jan 3 (common) / 4 (leap) Feb 28 (common) / 29 (leap) Mar 14 (Pi Day) — or "0" Apr 4/4 May 5/9 Jun 6/6 Jul 7/11 Aug 8/8 Sep 9/5...

Quiz: DOOMSDAY / The Doomsday Dates | updated: Jul 14, 2026

The "doubles": 4/4, 6/6, 8/8, 10/10, 12/12 — each even month has its doomsday on the day equal to the month number. April → 4/4 June → 6/6 August → 8/8 October → 10/10 December → 12/12 All five fall on the year's doomsday. They're the easiest to recall — month number repeated —...

Quiz: DOOMSDAY / The Doomsday Dates | updated: Jul 02, 2026

Split the last two digits with 12: doomsday = anchor + q + r + s (mod 7), where q = YY÷12, r = the remainder, s = r÷4. For year 19YY: q = how many whole 12s in YY. r = what's left over (YY − 12q). s = r ÷ 4 (whole part). Add q + r + s to the century anchor, mod 7. This small su...

Quiz: DOOMSDAY / Year & Century Anchors | updated: Jul 02, 2026

Wednesday. Anchor (2000s) = Tuesday (2). Year 01: q = 0, r = 1, s = 0 → 2 + 0 + 1 + 0 = 3 = Wednesday (year's doomsday). The date: August's doomsday is 8/8 (Wednesday). 15 − 8 = 7, 7 mod 7 = 0 → 15 August is also Wednesday. Check: 15 August 2001 was a Wednesday. ✓ Notice step 3...

Quiz: DOOMSDAY / Full Method & Practice | updated: Jul 02, 2026

Remember "I work 9-to-5 at the 7-11": the pairs 5/9 & 9/5, and 7/11 & 11/7. May → 5/9, September → 9/5 ("9-to-5") July → 7/11, November → 11/7 ("7-11") Each of these also lands on the year's doomsday. The mnemonic pairs a small month with a big day and vice-versa, so you get al...

Quiz: DOOMSDAY / The Doomsday Dates | updated: Jul 02, 2026

A single weekday, specific to that year, on which a whole set of easy-to-remember dates all fall. For example, in 2024 the doomsday is Thursday, so 4/4, 6/6, 8/8, 10/10, 12/12 (and several more) were all Thursdays that year. The genius is that once you know one year's doomsday, y...

Quiz: DOOMSDAY / What Is Doomsday | updated: Jul 02, 2026

1978 → Tuesday. Century anchor for the 1900s = Wednesday (3). Now YY = 78: q = 78 ÷ 12 = 6 r = 78 − 72 = 6 s = 6 ÷ 4 = 1 Sum: 3 + 6 + 6 + 1 = 16, and 16 mod 7 = 2 = Tuesday. Check: 1978's doomsday is indeed Tuesday. ✓ (So 4/4/1978, 6/6/1978, etc. were all Tuesdays.)

Quiz: DOOMSDAY / Year & Century Anchors | updated: Jul 02, 2026

A year is a leap year if it's divisible by 4 — except century years, which must be divisible by 400. 2020 → ÷4 → leap. ✓ 1900 → ÷100 but not ÷400 → not leap. 2000 → ÷400 → leap. ✓ 2023 → not ÷4 → not leap. This matters because leap years change the January and February doomsday...

Quiz: DOOMSDAY / Full Method & Practice | updated: Jul 02, 2026