You can simply let it pass or you can breed. It is good to clean your mare's legs, as when they are in heat their "discharge" can get on their legs, making their legs itchy and uncomfortable. If you find your horse moody when she is in season you can try different supplements specifically that are designed to help with the moodiness.
There isn't much you can do to stop a horse from coming into season. This is a natural occurrence.
Winter I should imagine.
You should cover a horse in WINTER
autumn
winter
in the spring time.
autumn-winter time
Winter
It is not common, but it is quite possible.
Usually you'd get your horse a new blanket in late fall or early winter. Or what ever the coldest season for your region is.
Horse fly season is from spring to fall. Horse fly season generally follows the last and first frost dates for an area. The flies take about one to two weeks to hatch after last frost.
I think in the Winter.