In summers, the maximum temperature crosses the 20 degrees Celcius mark in Spiti valley. During sunny daytime, you’ll feel comfortable just by wearing a t-shirt and trousers. It is always good to get wrapped in layers of clothes as the weather can change anytime in the Himalayas. Even if the sun starts shining again, you can always get rid of the outer layers of clothing.
Here is the packing list (summers) for Spiti valley:
T-shirts.  A windcheater/ rain jacket. Jeans/trousers.  Light woolen.  A cap/hat.  Sunglasses.  A pair of hiking shoes.  3/4 pairs of regular socks.  A water purifier and filtration bottle (avoid plastic bottle).  Personal hygiene essentials.  Sunscreen sunblock cream.
Spiti valley packing list (for winters):
A pair of good quality waterproof trekking boots. Three pairs of cotton socks. Two pairs of waterproof hiking pants. One pair of shorts Two pairs of fleece jumpers.  Two pairs of thermal tops and bottoms One down jacket. Headgear (1 beanie/skull cap, 1 buff, and 1 trekking cap).  A pair of thick gloves Personal hygiene essentials(alcohol-based gel hand sanitizer, soap, towels, toilet papers, baby wipes, etc.).  UV-block sunglasses. First aid kit. A water purifier and filtration bottle.