Today all players will get a free dart at the FarmVille Mystery Game! When clicking on the Mystery game you should receive a pop-up allowing one free try!
Official FarmVille Forum Community Manager Grimwell states:
FarmVille has a special gift for our players! Upon logging in, all players will receive 1 free Dart for the Mystery Game! Use your Dart to pop a balloon and win a Mystery Prize! Head to the Market and play our latest Mystery Game to receive your free Dart!