Frozen cake can taste just as good as the day it was first served if it is wrapped tightly with plastic wrap to prevent freezer burn. The trick to defrosting a cake with buttercream frosting is preventing condensation from forming on the frosting and keeping the frosting from coming off with the plastic wrap. As part of wedding traditions, the top tier of the wedding cake is frozen to be thawed out and enjoyed by the couple on their first wedding anniversary. Similarly, you might freeze leftover birthday cake if you can't finish it quickly.
Pull the cake out of the freezer the day before you plan to eat it. Leave it in its packaging.
Place the wrapped cake in the refrigerator for two hours. After two hours, remove the tight plastic wrap carefully so you don't drip condensation from the plastic onto the cake. The two-hour period allows enough time for the condensation to form on the outside of the plastic wrap, but not so much time that the buttercream becomes soft and sticks to the plastic wrap. If you remove the plastic right away, condensation forms on the frosting, making it runny.
Set the unwrapped cake in its box and place it back in the refrigerator to continue thawing overnight. If you don't have a cake box or a cake storage container, set the cake on a plate and cover it with an upside-down mixing bowl to keep it from drying out.
Move the cake to a counter or table about two hours before serving to bring it up to room temperature.