I did this wth our wedding cake except it was mud cake. I just took it out of the freezer and put it in he fridge 2 days before we needed it and it was fine. I would expect 24hours would be a normal time but i guess it depends on the size of the cake. Even if it's defrosted for an extra day it wouldnt really matter - better than finding it frozen in the middle.

Hope you have a lovely anniversary - our 2nd one is coming up in a few days!