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A day-of-week calculator determines what day of the week any given date falls on. The Doomsday algorithm (Conway) or modular arithmetic can compute this for any date in the Gregorian calendar.

Trin-for-trin guide

1. 1Use the formula: (d + m + y + y/4 − y/100 + y/400 + c) mod 7
2. 2d=day, m=month code, y=year, c=century adjustment
3. 3Or simply count forward/backward from a known reference date
4. 4JavaScript: new Date("YYYY-MM-DD").getDay() → 0=Sunday, 6=Saturday

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