cal/packages/lib/weekday.ts

type WeekdayFormat = "short" | "long";

// By default starts on Sunday (Sunday, Monday, Tuesday, Wednesday, Thursday, Friday, Saturday)
export function weekdayNames(locale: string | string[], weekStart = 0, format: WeekdayFormat = "long") {
  return Array(7)
    .fill(null)
    .map((_, day) => nameOfDay(locale, day + weekStart, format));
}

export function nameOfDay(
  locale: string | string[] | undefined,
  day: number,
  format: WeekdayFormat = "long"
) {
  return new Intl.DateTimeFormat(locale, { weekday: format }).format(new Date(1970, 0, day + 4));
}