I would say whenever you can get a good deal and I'm not sure there is a perfect time of year for that. If you're buying new, a 0% loan deal would be a big factor for me. If you're buying used you can spot and deal on them anytime. The biggest factor is how motivated the seller is to sell.
I have had luck in the fall to winter months especially if there is a new model replacing lasts years model that was more expensive. Dealership demo models with a few hours and you get the full warranty. Box stores don`t like to hold onto old stock, especially if it was a poor seller due to price. I saved $1200 on a brand new still the crate LT, the one replacing was only $400.00 more then what I paid for the one I got, I got a better motor too. Just got to keep a eye out for deals, even some used equipment with low hours and under a year old. I know were a nice like new home owner Scag Tiger Cub w/48" deck is with only 240 hours, for 44 hundred. Or a 1 year old same mower with 237 hours for 55 hundred, thats a grand off, still has balance of warranty. I don`t know what your looking for, big or little tractor or zero turn, there are deals out there.