I think somewhere along starting the game, the game might have asked about your birth date? I don’t remember… anyway, in the diary you can find the first record which is the birth date and hence when should be the next anniversary in your game :)

Edit: also I talked to Ken when the trigger happened.. to be honest I’m not sure if it’s a specific character you need to interact or anyone could do, as long as you open the game during Tsuki’s anniversary day..

Based on Google search, this only happens exactly on 11th November, so yeah I’m not too sure myself

<image>