Bhutan experience four different climatic season and depending on the season, you are advised to bring in the clothing accordingly.
Winter-(December, January and February) are cold in Bhutan and therefore warm clothing such as jacket, long sleeve shirts, warm inner wear will be of necessity. During the day time, especially when it is sunny, it will be bit warmer and light wear will come in handy.
Spring-(March, April & May) During this season, day temperature are moderate and a light cotton wear are perfect. However, due to a difference in altitude from place to place, a temperature varies and especially at night it could become cool. Therefore, it is worth bringing in warm clothing.
Summer-(June, July, August, and September) summer days are usually hot with temperature ranging from a minimum of 10 degree to a maximum of 30 degree depending on the altitude and places. A light cotton wear are perfect during this time. However, due to change in altitude, night can become cooler and a warm wear is recommended..
Autumn-(October & November) during this time of the year, days will be moderately warmer and night can become cold. A warm cotton wear, jacket, trousers and warm inner wear are required.
In addition, clothing as per your requirement, sunglasses, pair of shoes, hat, Umbrella, camera and toiletry items are recommended.