Generic term for dates not present in some/all calendar months?
Solution 1:
Hey I've thought of something I've seen in computer programming for these issues:
post28Days
ie, the days bigger than 28. you could also say "super-28 days" or similar.
I don't know if it's relevant to your situation.
Solution 2:
February 29 is called the "leap day" because it's not in every year. See http://en.wikipedia.org/wiki/February_29
That doesn't answer your question though. Sorry I don't have enough reputation points yet to just post a comment