Cherrapunji is one of the wettest places on earth, which affects when to go more dramatically than almost anywhere else. The monsoon runs roughly from May to September, delivering most of the annual 11,000+ mm of rainfall. During this period the waterfalls are at their most spectacular, the vegetation is impossibly lush, and the root bridges look their most dramatic — but the rain is constant and heavy, and the steep paths become very slippery.
October to March is the recommended window for most visitors. The skies clear, the views south over Bangladesh become possible, and the waterfalls are still flowing from stored water. The root bridge trek to Nongriat is far easier on dry paths. December and January are cool — warm layers are useful at the plateau elevation of around 1,300 metres.