Set a specific date for recurrent events

[Mantus667]

Although there are many options for setting recurring event criteria, it is not possible to create a recurring event for a specific date, e.g. 25th December for Christmas Day, which could fall on any day of the week but will always fall on the same calendar day. This is particularly important for anniversaries etc.

Cannot save a recurring event without specifying both a day of week AND monthly interval, neither of which should be mandatory if a specific date is given as requested above.

[Mantus667]

Related to #23