How We Calculate Dates

Every calendar, holiday and moon phase on this site is computed from published rules, not copied from a list. Here is exactly how each number is worked out, so you can trust the dates and know their limits.

Holidays and observed dates

US federal holidays follow federal law (5 U.S.C. § 6103). Fixed-date holidays like Independence Day (July 4) never move; floating ones are computed by rule, such as the third Monday of January for Martin Luther King Jr. Day. When a fixed holiday falls on a weekend, we apply the federal observed-date rule: Saturday shifts to the Friday before, Sunday to the Monday after. Observances like Valentine's Day and Halloween are shown on their true date and never shift.

Easter and related dates

Easter is a moveable feast — its date depends on the moon. We compute it with the standard Gregorian computus: Easter is the first Sunday after the first ecclesiastical full moon on or after March 21. Good Friday and other Easter-linked dates are derived from it.

ISO week numbers

Week numbers follow ISO 8601. Weeks start on Monday, and week 1 is the week containing the year's first Thursday (equivalently, the week with January 4). This is why a year can have 52 or 53 weeks, and why early January can fall in the last week of the previous year.

Working days

Working days are all weekdays (Monday to Friday) minus federal holidays that fall on a weekday. Holidays already on a weekend aren't subtracted twice. This matches how payroll and project schedules count business days. Your own employer may observe a different set, so we always show which holidays are included.

Moon phases

Moon phase dates are computed from the mean lunar cycle of 29.53 days measured from a known reference new moon. This is accurate to about a day. The exact moment of a phase depends on your time zone, so a date here may differ by up to one day from a local almanac. We note this on every moon-phase page.

Sources and standards

  • US federal holidays: 5 U.S.C. § 6103 and the Uniform Monday Holiday Act of 1968.
  • Week numbering: ISO 8601.
  • Easter: the Gregorian computus (anonymous Gregorian algorithm).
  • Moon phases: mean synodic month (29.530588853 days).
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Calculation FAQ

Are your holiday dates accurate?
Yes. They follow US federal law, including the observed-date rule for weekend holidays. Employers may observe a different set, so check your company's list for time off.

Why do your moon phase dates sometimes differ by a day?
They are computed from the mean lunar cycle, which is accurate to about a day. The exact time of a phase depends on your time zone.

Why does early January sometimes show last year's week number?
Under ISO 8601, week 1 is the week with the year's first Thursday, so a day or two of early January can belong to the final week of the previous year.