What is the leap-year rule you must apply?

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 dates (and the ⌊YY/4⌋ term already counts the leap days for the year calculation).