All those special dates for different countries could be a h... of work to program.
But many of those special dates can be found as tables on the net - or can be made manually.
I could suggest :
A setting in the recurring: Dates according to file xxx.yyThe file should/could contain a list of dates in a pre-defined format.
Then an event- serie could have a file-name with dates for that type of event.
Just an idea.
Yes, I was thinking that a workaround (other than manual input) would be to try to compute the dates using a spreadsheet application or a computer algebra system (CAS) and then import them as a CSV file.
For example, moon elongations happen approximately every half-orbit, so for example Ganymede is elongated from Jupiter approximately once every 3.57727648 days. So you only need to know the date and time of one elongation to estimate the dates and times of additional elongations. But LuxCal isn't really set up to show an event that recurs once every 3.57727648 days. That might be easier to import from a spreadsheet.
Another example would be when are your Martian birthdays? Terran (Earth) birthdays happen once every Terran year, but your birthdays on Mars are only once every Martian year (equal to once every 1.88082 Terran years). So if you go to Mars, you are actually younger (and weigh less) than on Earth, at least in Martian years (and Martian gravity). But you need to figure out the dates for an event that recurs once every 686.971 days (1.88082 years).