Change it whenever you get to it, usually last minute before machine is put in use, that's how it works here
I like to change oils just before the said piece of equipment goes into service. Changing oil at the end of season or just before winter has some valid points, but I don't think they contribute much to extending equipment/engine life. Our newer engines will go to 300 hours book says 600 hour interval, and that's it, it gets changed. I so t care what the book says, oil breaks down over use and time. Fresh oil often won't hurt anything and its cheap insurance, one hour of shop rate work is equal to an oil change.
Use good quality oil and change it within what you feel is a reasonable timeframe just dont exceed a drain interval. Last I checked they aren't giving tractors or anything with an engine away for discount prices, protect it.