Currently the calendar has "data-date" tags with the day of the month. It would be nice if there were "data-day" (or "data-dayofweek") which had 0=sun, 1=mon, ... 6=sat on each calendar day cell. This would help with add CSS themes to the calendar.

Some background: in Japan where we have our product, all our customers want to see Sunday in red and Saturday in blue. I would implement this as custom CSS.