Harvest season in Mendoza is in February and March, and while it's spectacular to see it in action, it does get pretty packed with tourists. During those months the wineries are in full swing, so getting tours can be trickier and you generally have to book well in advance. That said, if you're drawn to seeing the complete harvest and fermentation process live, it's totally worth going during that time - just be prepared to share the space with more people.
April and May are my favorite months to visit Mendoza without the harvest rush. Autumn is beautiful, the weather is perfect for touring wineries without dying of heat, and you can still find fresh grapes at many of them. Plus, prices are usually better and the winery owners have more time to give you proper attention. If what you're after is enjoying yourself at a leisurely pace, sipping wine in peace, and exploring several places in the same day, this time is ideal.
September is also a good option if you prefer spring. The weather is nice, there aren't as many people, and the wineries are less hectic than during harvest. The only thing is that in summer (December-January) you'd better skip it because the heat is unbearable for going from tour to tour, and besides it coincides with school holidays so there's a lot of crowding then too.