Winters (November to January): Winters in Himalayas are cold in general, but since Bir at an altitude of 1500m it is not as harsh as the higher towns like Manali. It is great when the sun is out, but evenings and nights require heaters or bonfire. You can also witness snow in Billing from mid December to mid January.
Summers (April to June): Days are warm, and evenings are pleasant. The sun feels too hot, but strong winds flow since its a broad valley and it is pleasant if you are sitting in the shade. Fans are required in the room during the day.
Monsoon (July to mid September): It rains heavily in Bir. Paragliding is closed from 1st July to 14th September, but it is one of the most beautiful times to visit since it is very lush and green all around. You must keep in mind that there will be more insects, and that going out becomes hard due to the rains.
Other seasons (February, March and September, October): Weather wise, these are the best seasons to travel to Bir. Its neither warm, nor too cold. It is pleasant weather all the time.