Mosquitoes mostly hatch in standing water during warm weather. In a typical year they get off to a quick start and then slowly taper down as the summer dries out. In a rainy summer, they will stay at high numbers. At least some mosquitoes will hang on until a heavy frost finally ends their season. Deer flies and horse flies are most prevalent during the warmer months.
Stable flies can be quite numerous during periodic hatches, mostly along shorelines.
Long pants and thick socks are the best defense. We haven't figured out any pattern to them yet. Make sure the zippers on your tent are closed really tight when they are around. Most everyone knows what they look like, the flying hypodermic needle. These look like overgrown fruit flies or a husky gnat, lots smaller than a house fly.