It's a tradition to decorate a flat ---- Christmas. on at in

The correct preposition to complete the sentence is "for."

So, the complete sentence would be: "It's a tradition to decorate a flat for Christmas."

If you're looking to choose from the options provided (on, at, in), the best choice would be "at," as in "It's a tradition to decorate a flat at Christmas." However, "for Christmas" is the most natural expression.