I recommend March if it's available. I did it March 1st 2019 and we were the first class in the new 2019 trucks. It was dry on the facility course but snowy and muddy in the mountains where they took us. The instructors commented it was some of the more challenging conditions they see all year, which enhanced the experience. They told us we were lucky we got to do it before everything melted off. The mud and snow stuff was actually the most fun. The "high speed" part is just so-so, as they only let you push the truck to around 5/10ths via speed restrictions.