The weather is often spring like. A light jacket and an umbrella will come in handy for the Summer and Fall months as the rainy season can extend from June through October. It can be cool to cold in December and January, so dressing in layers is recommended. The warmest and driest months are April and May, even so the thermometer will rarely break 90 degrees in the Valenciana.
There are several sites with up-to-date weather information. Our favorite site is Wunderground for complete Guanajuato weather.