I'm not understanding why an anniversary, even one celebrating 50 years of marriage, is more important than a birthday. Nobody in my family celebrates anniversaries (or, if they do, I'm never invited to the parties!). Same with my in-laws. I know my grandparents have been married for 50+ years, but as far as I know, there wasn't a big party for their 50th anniversary, and if there had been a party, it certainly wouldn't have been allowed to trump a grandchild's birthday.
Birthday parties, for my family, are the big deal. When my grandmother turned 80, I flew back to my HomeState to be there for the celebration, because she wanted as much of her family as possible to be there. When I celebrated my 30th birthday, my parents came out to CurrentState to celebrate with me. So I would be very upset if I was planning a birthday party for my hypothetical kids, and everyone deserted after an hour to go to an anniversary celebration that wasn't even announced for two months after the birthday plans were made.