Macarons are best kept in the refrigerator if they are not to be eaten immediately. If they get to stay there for a little while the flavors blends well together as well; the flavor of the filling infuse the macaron shells. I think macarons taste good cold straight from the fridge. If you don’t like the cold flavor, 15 minutes on the kitchen counter do the trick.
Macarons can’t remain long in the refrigerator. Two days is often the max, but it depends a little on what kind macarons you have made. The problem is often that they become crispy and dry, but the opposite can happen if you have a wet filling: the macarons turn chewy and sticky. If you are in doubt, a macaron should have a shell that is crispy on the outside and soft on the inside.
If you have made macarons that are not to be eaten at once, freezing is ideal. Macarons can be frozen unfilled and filled just before serving, but most can also be frozen with the filling. Let the macarons thaw 30 minutes in the fridge, and they are ready for serving. I always make too many macarons for myself, so that I can freeze some and eventually get a box in the freezer filled with many different types of macarons. It’s nice to be able to serve something that everyone visiting like! Remember that frozen goods get spoiled as well. Consume the macarons within three months time. (It should not be that hard … :)