Why do JWs celebrate anniversaries but not birthdays/Christmas?

My friend's grandparents are celebrating their 50th anniversary. They are really quite pleasant and nice people, but I find it strange that this anniversary is going to include many gifts and presents from friends and family. What is the difference between this kind of celebration and celebrating a birthday or celebrating Christmas? Everything is going to function virtually the same... it's just the name that has changed. Isn't it a little hypocritical?