Weekday calculation determines day of week for any date. Uses Zeller's congruence or similar algorithms. Essential for planning and historical research.
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Pro Tips
January/February: treat as previous year.
Leap years: divisible by 4 (except centuries).
Gregorian calendar: 1582 onwards.
Same date shifts 1 day per year (2 for leap).
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Fun Facts
"Gregorian calendar: 400-year cycle."
"13th more likely on Friday than other days."
"Doomsday algorithm: mental calculation method."
"Different cultures use different calendars."
Formula: Zeller's Congruence: day = (date + month + year + leap years) mod 7
