There is no real 'season' to visit Bukit Lawang. The rain season from October to February means torrential rain - but generally only in the afternoon/at night and it rains at moderate temperatures. Unfortunately the climate in Southeast Asia has been changing in recent years so accurate predictions are not possible! But we do not recommend travelling during Indonesian public holidays because hundreds of local visitors will crowd all Indonesia. This is our our Sumatra travel advisory because this is something that western tourists may find annoying because it can be a lot more noisy and hectic everywhere. If visiting Bukit Lawang on a Sunday it is wise to head for something quieter, such as jungle trekking, going to Landak river or to Tangkahan.