I just looked at a few calendar apps (Business Calendar 2, aCalendar+, DigiCal) and all 3 had the following repetition options:
* "every n days"
* "every n weeks" (select as many days of the week as you want)
* "every n months" (either same day, e.g. 13th, or every e.g. 2nd Monday)
* "every n years".
So it looks like the sort of repetition you are looking for is actually rather common.
If you want more, PimlicalA also let's you set up an irregular series of repeats. Of course you can also do that by duplicating an event in most calendar apps and then choosing the dates for each copy, but this is slightly more convenient if you do this often.