Trekking Seasons in Tibet are from mid of May till beginning of July, and from End of August to end of Sep.
For equipment, the following items of clothing are recommended for treks:
Good comfortable walking shoes or boots (be sure to wear them in first), cold-weather clothes, a flashlight (headlamp recommended), thick wool socks, long underwear,  T-shirts, cotton or woolen shirts with pockets, a rain coat, a down filled jacket, a sun hat, dark glasses, and sleeping bag. We supply the rest (sleeping mats, a woolen blanket, tents, kitchen equipment etc.).
On organized trek, we will provide cooks, yaks (somewhere horses), trekking equipment, including dinning tents, sleeping tents, mattress, washbasin, cooking all necessary utensils, vegetables, and main food.
If needed, sleeping bags are available for rental; and cold-weather gear as well as sleeping bags are easily available for purchase in Lhasa.