Traditionally, Catholics did not put up their Christmas trees until after noon on Christmas Eve. The same was true of all Christmas decorations. The purpose of the tree and the decorations is to celebrate the feast of Christmas; by putting them up early, we anticipate the feast, and Christmas loses some of its sense of joyfulness when it finally does arrive.
Most Christians today spend the entire season of Advent celebrating Christmas rather than preparing for it. It's natural, at this time of year, to want to enjoy the pleasures of hearth and home, and the greenery of the tree and the colors of the decorations add to that enjoyment. But we can get some of that, while still preserving the Advent season, by taking part in Advent activities and devotions, such as the Advent wreath and Advent calendars.
Of course, if you wait until Christmas Eve to purchase your Christmas tree, you might end up with a sad, spindly looking stick like the one that Charlie Brown brings to the Christmas pageant in A Charlie Brown Christmas. (On the other hand, you might also get your tree at a very low price, or even free!) But holding off on purchasing a tree until Gaudete Sunday, and then decorating it as late as possible, is a reasonable compromise.
Even if circumstances make it necessary to put up the Christmas tree earlier in Advent, we can still maintain some sense of the Advent season by not lighting the lights until Christmas Eve, or by putting out our most precious decorations (and perhaps the star for the top of the tree) only on Christmas Eve. Such practices increase the sense of expectation, especially among young children, and make Christmas Day all the more joyful.