< day-of-the-week > &optional < format > )
Returns the name of `day-of-the-week`. The parameter should be a number between 0 and 6 where 0 represents Sunday and 6 repressents Saturday. The optional format argument can be either :long or :short. In the latter case, the return string will be of length three; in the former it will be the complete name of the appropriate day.