Spring is the best time to visit San Antonio. March through May brings warm days (70s–80s°F), wildflower season in the surrounding Hill Country, and Fiesta San Antonio in April — ten days of parades, street parties, and events marking the anniversaries of the Alamo battle and the Battle of San Jacinto. Fiesta is one of the largest festivals in Texas and well worth timing a trip around.
Summer (June–August) is brutal: temperatures regularly exceed 100°F and the humidity makes it worse. The River Walk becomes oppressively crowded. If you visit in summer, spend mornings at outdoor sites and afternoons in air-conditioned museums.
Fall (September–November) offers good weather and lighter crowds once school starts. Winters are mild — January highs around 61°F — and the city is easy to explore without the summer heat. The River Walk's Christmas lighting is genuinely spectacular.