Rainy Season
So for my money, winter is not a good time to come. Nor are April and May comfortable months. They're hot, dry, and dusty. Some tourist-related businesses give up and close during the spring for lack of customers.
No, the sweet spot is during the months of June through October, our rainy season. Scattered clouds fill the sky, moderating the intense sun and lowering temperatures.

Many afternoons feature a thunderstorm or two. They're often intense, but rarely last more than an hour, after which the sun breaks through, the streets are fresh and washed, and the climate is ideal. In these months we get three quarters of our annual 23" of rainfall.
We also get nice sunsets.

Some are put off by the idea of vacationing during the "rainy season." The term, though, is relative: the eastern half of the the US from the great plains to the atlantic seaboard and all of the Pacific Northwest receives roughly double the precipitation we do. Nobody's vacation is going to be ruined by visiting San Miguel during the summer.
We welcome many Norteamericano visitors in July and August. In June, September and October we smugly enjoy uncrowded restaurants and Mexico's best climate as we anticipate the arrival of next winter's crowd of shivering Canadians.
Of course, we Sanmigulenenses will be tanning on the beach at Yelapa by then.