Super cheap flights to Paris can and do pop up unannounced all the time, so don't get in the habit of only shopping for fares when you expect they'll be cheap. Otherwise, you risk missing out on those random sub-$300 fares that we so often see, even from the West Coast. Generally, you'll find the lowest fares for travel between Thanksgiving and Christmas, and again in winter. September and October is also a great time to go, as fares tend to drop quite a lot after the summer rush.