Day of Week Calculator

Every calendar date falls on a specific day of the week — Monday through Sunday — cycling every seven days. Determining the weekday for any date uses modular arithmetic anchored to a known reference. The Gregorian calendar reform (1582) and leap-year rules (divisible by 4, except centuries unless divisible by 400) ensure the pattern repeats every 400 years (146,097 days = 20,871 weeks exactly).

Take July 4, 1776 — the United States Declaration of Independence. Applying Sakamoto's algorithm or Zeller's congruence: July 4, 1776 was a Thursday. A modern check: July 4, 2024 was also a Thursday. Adding 365 days (one common year) advances the weekday by one, so July 4, 2025 is a Friday and July 4, 2026 is a Saturday.

For mental estimates near today: each common year advances weekdays by one (365 mod 7 = 1); each leap year advances by two (366 mod 7 = 2) when the February 29 extra day falls within the span. From January 1, 2025 (Wednesday) to January 1, 2026 (Thursday) adds one weekday step. Spanning a leap year's February from March 1, 2023 to March 1, 2024 advances by two weekdays because 366 days elapsed.

Knowing the weekday helps schedule recurring events, verify historical dates, plan travel around weekend pricing, and align ovulation or fertility tracking with calendar days. The day-of-week is independent of time zone for calendar dates — midnight boundaries may differ for UTC vs local, but a calendar date like "March 15, 2026" has one weekday globally.

Enter any date to instantly see the day of the week. Use it to confirm appointment scheduling, check whether a historical event fell on a weekend, or plan future events that must land on a specific weekday.