When is flea season? Flea season varies. Flea season really is any time of the year where it's hot, and dry out usually, that's when the fleas get really bad. For people living in like say the northern hemisphere and temperate regions, that's generally going to be like from March, as early as March, okay, through April, May, June, July, August, and then usually ending sometimes in September though it can stretch into October if necessary. Now, the key here is flea season lasts as long as there are fleas. So, if you have a house and you're keeping that thermostat to be controlled, and it gets to be say, I don't know, December, and you still got fleas in your house 'cause you haven't treated, well guess what, it's still flea season for you. So, really it has to do with temperatures, dryness and humidity mainly. The season while it can have some effect on it, isn't all that important.