Querétaro has a mild highland climate at around 1,800 metres altitude, and is pleasant to visit year-round. The dry season runs from October through May, which is the most comfortable period — warm sunny days (20–28°C), cool evenings, and no rain. The rainy season from June through September brings afternoon showers most days, usually short and cooling, though the surrounding countryside turns vivid green and the city feels cooler and fresher.
September 15 is worth seeking out if possible: the night of the Grito de Independencia, when the city governor re-enacts La Corregidora's historic warning from the Casa de la Corregidora balcony and the Plaza de Armas fills with thousands of people waving flags and calling out "Viva México!" It is one of the most exhilarating expressions of Mexican national identity you will encounter.
The Chiles en Nogada season — when this Poblano classic of chiles stuffed with picadillo and covered in walnut cream sauce is available — runs roughly from August to October. It is worth planning a visit around.