Rosetta Code

Day of the week

Compute weekdays for fixed Gregorian dates with Zeller's congruence.

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Sourcerosettacode/popular/day_of_the_week.vibe
def weekday(year: int, month: int, day: int) -> string
  adjusted_year = year
  adjusted_month = month

  if adjusted_month < 3
    adjusted_month = adjusted_month + 12
    adjusted_year = adjusted_year - 1
  end

  century_year = adjusted_year % 100
  zero_based_century = adjusted_year // 100
  code = (
    day +
      ((13 * (adjusted_month + 1)) // 5) +
      century_year +
      (century_year // 4) +
      (zero_based_century // 4) +
      (5 * zero_based_century)
  ) % 7

  weekday_names.fetch(code)
end

def weekday_names -> array<string>
  ["Saturday", "Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", "Friday"]
end

def run -> array<{ date: string, weekday: string }>
  [
    { date: "2026-03-08", weekday: weekday(2026, 3, 8) },
    { date: "2024-02-29", weekday: weekday(2024, 2, 29) },
    { date: "2000-01-01", weekday: weekday(2000, 1, 1) }
  ]
end

Output

Click Run example to see the result.

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