next_day returns the date of the first weekday named by char
that is later than the date date. The argument char must
be a day of the week in the date language of your session, either the
full name or the abbreviation. The minimum number of letters required is
the number of letters in the abbreviated version. Any characters
immediately following the valid abbreviation are ignored. The return
value has the same hours, minutes, and seconds component as the argument
date.