Day of the week

# title: Day of the week
# source: https://rosettacode.org/wiki/Day_of_the_week
# category: Rosetta Code
# difficulty: Intro
# summary: Compute weekdays for fixed Gregorian dates with Zeller's congruence.
# tags: popular, dates, math, calendars
# vibe: 0.2

def weekday(year, month, day)
  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[code]
end

def weekday_names
  ["Saturday", "Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", "Friday"]
end

def run
  [
    { 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