I agree with backstage (and others) who have said, “it depends.”
Like with any weather situation, it totally depends on what your horse is conditioned to. Here in the temperate northwest, I see people start posting that they’re canceling lessons and rides when the temperatures drop below 30 degrees or above 95/100 degrees.
Can you imagine how little would get done in the SE if you didn’t ride any day where the temp hit 95 degrees (with virtually no humidity)? And same story for what people in the midwest or far north would do if they didn’t ride below 30? But the horses aren’t conditioned to it here, so when we get unusual temp runs, we don’t ride.
So if your horse has just moved to Boston with you and is from Florida, then I would err on the side of caution and probably stick to a higher range than you might hear suggested by fellow (local) riders. If the horse is from the area or has lived in a cold climate for a while, then the range of comfort might be more extreme than you yourself are comfortable with.