How do you use a doomsday date to find a target date's weekday?

Take the nearest doomsday date in the same month, find the day difference, reduce mod 7, and step that many days from the doomsday.

Suppose the year's doomsday is Monday and you want October 31:

• October's doomsday date is 10/10 (a Monday).
• 31 − 10 = 21, and 21 mod 7 = 0.
• So 31 October is also Monday.

For October 25: 25 − 10 = 15, 15 mod 7 = 1 → one day after Monday = Tuesday. Always count from the month's own doomsday date to keep the number small.