You certainly can freeze a decorated cake up to several months (although I've never kept one frozen for more than a month). Just box it up and double wrap with plastic wrap and a final covering of foil. When you get ready to thaw, put it in the fridge for a day and then on the counter a couple of hours before serving. DO NOT unwrap until it has thawed completely so any condensation will be on the wrapper instead of the cake itself.
It might depend a bit on what kind of cake and icing used. And some colors could run when defrosting it. I've had both experiences. I use mostly box cake mixes and b'cream icing. Colors run if they are freshly colored. Make you colors several days ahead of time so they have time to 'develope'/meld to prevent colors running. Otherwise what cakefan 92 said is right.
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