Day of the Week from a Birthdate

This is a compact version of Gauss’s weekday calculation: it turns a date into a number from 0 to 6 that matches Sunday through Saturday. The idea is to add up the day of the month, the month’s offset within the year, and the leap-year contributions from the year and century, then reduce the total modulo 7 because weekdays repeat every 7 days. The special month numbering starts the year in March so that February can be treated as the last month of the previous year, which makes leap years fit cleanly into the arithmetic.

Use this when you need the day of the week for a historical date, birthdate, or anniversary without a calendar lookup. It is especially useful in maths puzzles, date calculations, and quick checks in history or genealogy work.

• Forgetting to shift January and February into the previous year and using the wrong month number
• Mixing up the weekday codes, especially assuming 0 means Monday instead of Sunday
• Using the full year 1990 in the y term instead of only the year within the century, 90