The pilgrimage season is from March (Easter) until October, with the pleasantest weather in May, June and September. The summer months can be extremely hot, especially on the meseta, the high and very exposed plain between Burgos and León on the Camino francés, and on the more southerly sections of the Via de la Plata. Seville is especially hot, with summertime temperatures in the 30s and regularly reaching the high 20s in May and October.
July and August are also the most crowded months on the Camino Francés, although this route is becoming increasingly busy in spring and autumn too.
Winter walking brings a different set of challenges altogether, please see What about going in Winter? for more details.