Are there anniversaries that are suppressed because they are politically/socially/religiously "inconvenient"? Do those who want to commemorate these go underground to do so (literally maybe?) or are they watched by the authorities to make sure they can't do that?
If an anniversary is important to Character A but not to Character B, how does that affect the relationship between them? Where there are state approved anniversaries, who carries out the celebrations officially?
Lots of story ideas there!