code: func builtinDayName(args []types.Datum, ctx context.Context) (types.Datum, error) {
  	d, err := builtinWeekDay(args, ctx)
  	if err != nil || d.IsNull() {
  		return d, errors.Trace(err)
  	}
  	weekday := d.GetInt64()
  	if (weekday < 0) || (weekday >= int64(len(types.WeekdayNames))) {
  		d.SetNull()
  		return d, errors.Errorf("no name for invalid weekday: %d.", weekday)
  	}
  	d.SetString(types.WeekdayNames[weekday])
  	return d, nil
  }
docs: // See http://dev.mysql.com/doc/refman/5.7/en/date-and-time-functions.html#function_dayname