Glad to hear from you. During the wintertime, it will be warm during the day; it will be about 15°C to 20°C at the beginning of the Langtang Valley trek. And at the highest point in Kyanjin Gumba will be 5 to 10 degrees Celsius during daytime and night and early morning will be -5°C to -15°C outside. But it will depend on the following day's weather conditions too. Normally, the weather is perfect during September, October, and November in autuman and Spring (March, April, and May). But the high Himalayas weather conditions are unpredictable. It might rain or snow at any time. Let's think good then everything goes perfectly. If you have a -5°C to -10°C sleeping bag it is good enough, if you feel it is not enough every tea house provides a blanket as well.