If a holiday contains a double quote character in either the title or description a javascript error is thrown which prevents the datepicker fromm loading. To recreate: 1 - define a holiday (single or repeated) with a double quote in either title or description 2 - Date picker disappears and you cannot edit the holiday
Confirmed. Set the title to [My "quoted" holiday] and the calendar vanishes. To make the calendar appear again, a database administrator has to remove or to edit the entry in table special_holidays.
Closing as a duplicate of 18689 (which is newer, but Marc Véron helpfully added a number of "See Also" links to that one). *** This bug has been marked as a duplicate of bug 18689 ***