Dozens of times each year I am asked the very same question. "When is the best time to mail out my postcards?" Although it's nearly an IMPOSSIBLE answer. I always give my best advice. Depending on where you live and the climate you're in, there is a "sweet spot" on when people are thinking about "lawn care," "mowing" or "springtime" in general. Your marketing should be focused around that "sweet spot."
Now let me say, I don't feel you should put all your eggs in one basket