Disabled dates get shifted when firstDay is 1

[NateWr]

If the datepicker's firstDay setting is set to 1 (or presumably anything other than 0), it causes an error with disabled weekdays on the edit booking form. When opening the booking form from the edit link in the admin panel, the disabled weekday is offset by 1 (or presumably the value of firstDay). Closing the booking form completely then re-opening it from the edit button causes the disabled weekday to be offset again, with the offset value compounding with each open/close of the entire booking form.