Day of Week Algorithm (Mental Calendar)

Problem

Figuring out what day of the week a date falls on requires a calendar.

Solution

Use Zeller's congruence simplified: Learn anchor days for each month and count forward/backward.

Benefit

Impress others by calculating any date's day of week mentally.

Example

For 2024 (leap year):
Jan 1 = Monday (anchor)
Feb 1 = Thursday (+31 days = +3 days)
Mar 1 = Friday (+29 days = +1 day)

What day is Feb 14, 2024?
Feb 1 is Thursday
14-1 = 13 days forward
13 ÷ 7 = 1 week + 6 days
Thursday + 6 = Wednesday