Yes you can ice a cake with buttercream the day before as long as you make sure to refrigerate the cake overnight after adding the buttercream icing to the cake.
You can also make the cake the day before and then just add the buttercream icing to the cake the next day after the cake warms up after being refrigerated.
For the best cake with buttercream icing or any other icing it's best to put the icing on the same day you plan to serve the cake.
But you can add the buttercream icing the day before and make it ahead of time so that it's ready.
But if possible I would just apply the buttercream icing to the cake the next day because after you remove the cake from the refrigerator it should only take a few minutes or so to add the icing to the cake.
The cake will turn out better and the icing will be much fresher on the cake.