Work out the doomsday for 1978 step by step.

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.)