Javascript Holidays using built-in date function (static & floating)

javascript-holiday-validation.js

function isHoliday(date: Date) {
  const holidays = {
    MD: {
      // Month, Day
      '1/1': "New Year's Day",
      '7/4': 'Independence Day',
      '11/11': "Veteran's Day",
      '12/25': 'Christmas Day',
      '12/31': "New Year's Eve"
    },
    MODW: {
      // Month, Occurence, Day of Week
      '1/3/1': 'Martin Luther King Jr. Day',
      '2/3/1': 'Presidents Day',
      '5/L/1': 'Memorial Day',
      '9/1/1': 'Labor Day',
      '10/2/1': 'Columbus Day',
      '11/4/4': 'Thanksgiving Day'
    }
  };
  const dayOfTheMonth = date.getDate();
  const month = date.getMonth() + 1;
  const dayOfTheWeek = date.getDay(); // 0 - 6, Su -> Sa
  const lastDayOfTheMonth = new Date(date.getFullYear(), date.getMonth() + 1, 0).getDate();
  const isLastOccurrence = dayOfTheMonth + 7 > lastDayOfTheMonth;
  let currentOccurrenceDay = dayOfTheMonth,
    occurrence = 0;
  for (currentOccurrenceDay; currentOccurrenceDay > 0; currentOccurrenceDay -= 7) occurrence++;

  return !!(
    holidays.MD?.[`${month}/${dayOfTheMonth}`] ||
    (isLastOccurrence && holidays.MODW?.[`${month}/L/${dayOfTheWeek}`]) ||
    holidays.MODW?.[`${month}/${occurrence}/${dayOfTheWeek}`]
  );
}