Bursa has a temperate climate with hot summers and cold winters — more so than Istanbul, owing to its position at the foot of Uludag. Spring (April–June) and autumn (September–November) are the best times to visit: pleasant temperatures, few crowds, and the fruit and chestnut harvests in autumn.
Summer (July–August) is hot in the city and can be crowded at weekends with day-trippers from Istanbul. Winter (December–March) is cold and sometimes snowy in the city, but Uludag is busy with skiers; if you want to ski, book accommodation on the mountain well in advance. The cable car can close during very bad weather.
The Silk Cocoon Market at the Koza Han operates in June and July, when Bursa's silk farmers bring their cocoons to sell. It's worth timing a visit around it if you're interested in the silk trade. The Bursa International Music Festival runs in June.