The pat answer would of course be during the hatch. But, the practical answer is anytime you can break free and fish. The insects flies mimic are small so fish need to feed constantly. Terrestrial patterns are bonus meals of opportunity. Mosquito patterns can be fished at those times of day when mosquitos are buzzing around. Same for fly and gnat patterns. Minnow patterns are also targets of opportunity for larger fish. Just use your powers of observation.

Sourdough Dave and 16country boy are right. Get on the water anytime you can. Poppers, terrestrials, streamers, they all work. Just go with your observations or past experiences. Also, ask around about fishing conditions on your local waters. Remember, rain or shine, fish are hungry and have to eat all year.