class Date

def find_beginning_of_week!(week_start)

Returns week start day symbol (e.g. +:monday+), or raises an +ArgumentError+ for invalid day symbol.

def find_beginning_of_week!(week_start)
  raise ArgumentError, "Invalid beginning of week: #{week_start}" unless ::Date::DAYS_INTO_WEEK.key?(week_start)
  week_start
end