I learned this at MathCamp from Conway himself! The mnemonics he taught us for the “doomsdays” were:
> Even months aside from February, same date as the month, so 4/4, 6/6, 8/8, 10/10, 12/12.
> “Working 9 to 5 at 7-11”: 5/9, 7/11, 9/5, 11/7
> Jan–Mar are a bit arbitrary: Feb is the last day of the month (so 2/28 or 2/29), March is the 0th day (same), and Jan is 1/31 or 1/“32” depending on if it’s a leap year.
> Even months aside from February, same date as the month, so 4/4, 6/6, 8/8, 10/10, 12/12.
> “Working 9 to 5 at 7-11”: 5/9, 7/11, 9/5, 11/7
> Jan–Mar are a bit arbitrary: Feb is the last day of the month (so 2/28 or 2/29), March is the 0th day (same), and Jan is 1/31 or 1/“32” depending on if it’s a leap year.