The fever meter is indicated in the top left by a shamrock. It is filled by finding shamrocks outside around town or by finishing quests. Once the fever meter is filled, it can be manually activated for a coin bonus. Activating FEVER will cause your garden plots to rapidly produce coins, and balloons will flood the screen which can be popped for coins as well.
During a fever, party music will play and all pets onscreen will dance happily. In the background, you will see the sky changing colors.