for Day.rb, a reference to valid day strings

parse_day_argument.rb

def parse_day_argument(day)
  case day
  when 'su', 'sun', 'sunday', '0'
    return :sunday
  when 'm', 'mo', 'mon', 'monday', '1'
    return :monday
  when 'tu', 'tue', 'tues', 'tuesday', '2'
    return :tuesday
  when 'w', 'we', 'wed', 'wednesday', '3'
    return :wednesday
  when 'th', 'thu', 'thur', 'thurs', 'thursday', '4'
    return :thursday
  when 'f', 'fr', 'fri', 'friday', '5'
    return :friday
  when 'sa', 'sat', 'satu', 'satur', 'saturday', '6'
    return :saturday
  else
    raise ArgumentError, 'Could not parse day argument! Check your day glyphs.'
  end
end