Assisi works well year-round, but the experience changes dramatically with the season.
April to June is the best window — warm but not hot, the surrounding hills green with wildflowers, and the religious calendar full of processions. The week of 4 October (Feast of St. Francis) is the largest pilgrimage of the year and brings the town to standstill, in a good way if you want the atmosphere and a bad way if you want to walk through it quickly.
July and August are hot (highs around 30 °C) and crowded; the basilica fills with coach parties and the side streets get oppressive in the middle of the day. Even so, evenings are pleasant and the long light over the Umbrian plain is unforgettable.
September to early October has the best of both worlds — warm days, cooler nights, the grape and olive harvest underway in the surrounding hills.
November to March is the quiet season. Some restaurants close, but the basilica is essentially empty and you can spend an hour with the Giotto frescoes alone. Heating in old stone buildings is unreliable, so bring layers. Christmas at the basilica is particularly atmospheric.