The number cake is at its best in the day of the preparation, but yes, you can prepare the tarts a day ahead, store them in the fridge. Also prepare the cream a day a head or in the day of serving and store in the fridge. Then, when ready to serve, pipe the cream over the tarts and assembly the cake and serve. But I would not recommend making the whole cake ahead of time, as the tart become soggy after some tome.