My filly was weaned at 3 1/2 months, simply because she was taking a lot of weight off of momma. She did perfectly fine, and was weaned with her 4 other herd mates the same day, so they all went through it together and did fine. One of the fillies she was weaned with was just 3 months, and she did fine as well, she had a little trouble eating for a couple days, but now she is perfectly fine, and Rumour's best friend!
It really depends on your mare, then situation, and the weather. I wouldn't wean on one of the hottest weeks, or months, because the mare and foal are working themselves up so much, and the heat on top of that would be threatening to their health. We weaned on a cool, misty day.