Because fresh fruits adorn the tops of these tarts, it's recommended that you wait until up to a few hours before adding the fruit. The longer fruit sits the juicer it gets, and it could result in your tarts having soggy bottoms. If making these ahead of time, the cookie cups can be baked and stored in an airtight container for a day or two in advance. You can also make up the cream filling the day before and store it in the fridge, covered tightly with plastic wrap an airtight bowl until you are ready to fill the cups. If you are really needing a short cut, you can pre-slice all the fruit the night before and keep it in fridge until you're ready to decorate the tarts.