Spring and autumn are the best times to visit Alberobello. April and May bring warm days, wildflowers in the Valle d'Itria, and manageable crowds. September and October are similarly pleasant, with the added bonus of truffle and grape harvest activity in the surrounding countryside.
Summer is hot, crowded, and expensive. The lanes of Rione Monti become genuinely packed in July and August, and the trulli that look so mysterious at dusk fill with tour groups by 10am. If you must visit in summer, arrive early and leave the afternoons for the beach towns on the Adriatic coast, an hour away.
February is worth knowing about for a specific reason: almond trees bloom across the Valle d'Itria, and the combination of white blossom and white-tipped trulli, often in crisp winter light, is striking. The valley around Locorotondo and Cisternino is particularly good for driving or cycling through the orchards.