Mathura is most intense during Holi and Janmashtami, when the city and the wider Braj region fill with pilgrims. These are the most memorable times to visit if you want the festival atmosphere, but they also mean crowds, higher prices, and slower movement through the old city.
October to March is the easiest season for general travel, with cooler weather and better walking conditions. May and June can be punishingly hot on the plains.