yizkor and grandparents
Mourning non-Jewish relatives is a little strange anyway. The practice that has evolved for me, though it's kind of odd I suppose, is that I say kaddish on the appropriate dates for the two grandparents who died since I became religious, but not for the other two. One of the other two turns out to have died three days (on the Jewish calendar) before one of the others, so I mentally include her at the same time. The fourth, my paternal grandfather, died when I was a child and I don't even know the date. And I liked him, and I feel funny about leaving him out of the "formal" observance completely.
This Shavuot during yizkor it struck me: I don't know the date, but I do know that he died in May, and Shavuot is usually in May (or early June), and I'm going to be there for yizkor anyway... so it seems logical that, for purposes of private observances (e.g. memorial candles), I should observe my grandfather's yahrzeit on Shavuot. That also creates some degree of parity -- each of the grandparents who died long ago is getting added onto something else, but no one is getting forgotten. (Not that I would forget them in any case -- but I mean formally.)