Each week, you'll need to take a picture of your Downtown Expansion (make sure enough details are around to show that you're actually in Downtown and not your overall city) and then post it on the game's official forums. From there, you'll have a chance to win 100 City Cash, and the admiration of your peers as the winning entry is featured on the game's fan page.
All players will have a chance to vote on each week's finalists, giving you a chance to have a final say in (perhaps) receiving your own reward. However, you can't win any free City Cash if you don't enter, so make sure to check out the rest of the official rules over on the game's forums! Good luck!
Will you enter the CityVille Downtown of the Week contest, or have you yet to create anything prize-worthy in your Downtown expansion? Sound off in the comments.