What Season Does A Horse Need A Blanket ....fall....winter....summer...spring...?

I agree with the third answer with one exception. If you have a horse that's breed is use to warm weather then you probably need a blanket in the winter.
There are other uses to blankets as well. I use a fly sheet for when the flies are really bad and I use the same fly sheet when it is icy because we get a lot of ice in the winter and instead of using a thick heavy blanket to keep him warm I use a fly sheet just to keep him dry he can stay warm on his own.
Maybe if you have a field with no trees or no shelter at all, get a lean-to. An open type of building with 2 sides and a roof where 2 sides are open. This can be used as a wind break or to get in out of a really bad storm. But don't be surprised if its 130 degrees and the horse is in the sun (flies), or if its -10 degrees and the wind is blowing and your horse has his but to the wide and is just standing there. All these are what a horse does. The purpose to a lean-to is to give your horse an escape just in case he wants to use it!!! Options are always good and happy horse loving

If you're talking about a blanket for warmth, mostly Winter. But don't blanket your horse unless he needs it! Unless it's a Arab, TB, or other horses that usually don't grow a coat, he probably doesn't need one. If your clipped your horse, he will need one though. See how his coat grows in, and see if he's shivering, and his ears are cold.

There are other blankets though, mostly for Summer, like fly sheets and coolers.

A woman named Sue lives down the road and owns a nice horse farm. Sue tells us that she NEVER puts the horses in the stable or puts blankets on them. Why, you ask? Because, horses in the wild, do not need much shelter, do not wear blankets. Sue tells me that whenever she boards a horse, the owners pay extra for the horse being put in a stable. Put a horse outside and keep it there, a horse will be content in any weather, because no horse does not have any wild in him.

I blanket my horse in late November.   The only reason I do so is that winters here in Michigan are very cold and Sunny is on pasture board and my horse does not tolerate wind well..   She hates stalls, and they are really quite unnecessary. Having said that, there are many people who do not believe blanketing is necessary in any weather.   All horses, regardless of breed, develop a winter coat.   I have had Arabs and thoroughbreds who did not need blanketing.

Around autumn to winter, depends on what breed your horse is.