In the months of October, November, December, January and February, mornings and evenings will be cold. You will have to being in warm clothes (thick overcoats not necessary). While the months of February, March, April, May, June, July, August and September the days are warmer. June, July and August will be little wet and some rain gear would be necessary.
Due to wide range of temperature and climatic conditions, it is advisable to bring appropriate clothing.
Sunglasses/spare glasses or contact lenses, pair of casual shoes, trekking boot if you choose bhutan treks, washing kit, shaving kit, towel, hat, umbrella, camera, film and accessories, maps, insect repellent, hand cream, small sewing kit & safety pins, torch or flash light with spare batteries, mirror, sun screen cream, lip salve or soluble aspirin, antiseptic cream, preparation for the relief of sunburn. You may not be tuned to the Asian drugs so it is always better to bring own brand.