Macarons are notoriously difficult to make, and so it really does pay to make them ahead of time and finish them off when needed. Freezing them is a great way of being prepared and it is so easy to do.
After you have baked the shells, let them cool then wrap them and the baking sheet/tray in cling film and freeze.
When you need them, let them defrost for 30-40 minutes at which point you can fill them as desired.
They should last in the freezer for up to 3 months (that's if they last long enough).