Yes, usually it's around the actual anniversary of graduation but it really doesn't matter, whomever takes the lead in planning usually decides the dates based on their schedule, availability of the reunion site, and pricing/deals. It never works out perfectly for everyone. And I don't think you're being ungrateful, BTW, you have a value question.
My class graduated many moons ago. We had a 5, 10, 15 and 25, then nothing. The person who usually did the coordination took very ill. Almost 2 years ago my friend took the initiative to start a Facebook page to help find the remaining classmates and have a reunion, we've located about 160 and 130 of us are on the FB group page. Since we'd totally missed a 40 year reunion we did one last year in June, a 41st. We were originally from SoCal and many weren't able to travel back here, so we're having a semi-reunion more centrally located in the U.S. in Branson, MO this October, 42 years 5 months after graduation, about 30 so far are coming. And in our group at least three people concur when setting a date, we had one woman pushing for an earlier one last year, she didn't say why, she was dying from breast cancer.
The class behind me had their reunion 40 years, 6 months after graduation, the one after that at the same time, 39 years, 6 months.
Hopefully my rambling makes a point, in my opinion it's the actual reunion, not the timing, that's important, especially as more years pass :)