From what I have read, fall is the best time. Use a cover crop such as oats, which will give you fall attraction on their own, around the 1st of September. Supposedly fall planting results in less weed competition for the clover. That said, I planted my Clover Plus in the spring and had pretty good success.
Frost seeding in late March is supposed to work well also. My Clover Plus is about the first thing to green up in the timber and seems to get a jump on the competition from weeds, which usually catch up a little by early summer.
