Wells is worth visiting year-round, but summer (June to August) is by far the busiest period — parking becomes difficult and the cathedral close fills up on weekends. The light in July and August is beautiful on the west facade of the cathedral, especially in the late afternoon.
Spring (April and May) and early autumn (September and October) are better for a quieter visit. The surrounding Somerset Levels and Mendip Hills are at their best in spring, when the meadows flood with wildflowers, and in autumn when the apple orchards are heavy with cider fruit. Winter is rarely harsh — Wells sits at low altitude and is sheltered — and the cathedral is spectacular in December, with carol services and a small Christmas market in the Market Place.
The Wells Cathedral Festival, held in late summer, brings music and events to the cathedral and city. Check the cathedral website for the current programme if your visit coincides.