What are the doomsday dates for January and February — and why do they depend on the year?

February's doomsday is the last day of February (2/28, or 2/29 in a leap year); January's is the 3rd, bumping to the 4th in a leap year. Both land on the same weekday, so they move together.

• February: the last day — 2/28, or 2/29 in a leap year. It is the leap boundary, so there is no number to memorise.
• January: the 3rd — but a leap year leaps it to the 4th. Hook: a leap year is divisible by 4, and January's doomsday jumps to the 4th.

Why they move together: Jan 3 → Feb 28 and Jan 4 → Feb 29 are each exactly 56 days = 8 weeks apart, so the two anchors are always the same weekday. The leap day sits at the end of that stretch, so when it exists both anchors slide one day later.