Every year starting in February the flies come swarming.  I have purchased the smaller versions of traps like these, but this year decided to get the gallon hoping it would keep me from having to empty it out as often.  I set it up today, and instead of waiting as the instructions said I stuck it outside on the ground where the flies are. Less than 5 minutes later there are already lots of flies in the jar and many more standing in line waiting to go in.