April to Mid-May is a great time. April marks the beginning of tourist season in Ladakh when the place begins to warm up with tourist activity. Restaurants open, hotels and guest houses start to welcome guest and various camps at sites such as Pangong Tso and Tso Moriri Lake commence to set up. Usually by the second week of May, the crucial Srinagar-Leh highway opens up leading to a flurry of tourism energy into Ladakh. By late April, all the lakes including the Pangong Tso and Tso Moriri begin to melt, uncovering the beautiful colors and fabulous sights. In order to welcome more guests, hotels and guesthouses offer heavy discounts, benefitting all the travelers who happens to be present here around this time. However, both Khardung La and Chang La passes are still under the snow and it’s a memorable experience in itself to pass through them.
If you want to go by road from Manali then May end of June. It's not possible to travel to Leh by road from Manali until Rohtang pass is open for general public.
If you flying to Leh, then visit in May.
September is one of the best time to go to Leh as this time offers some of the best sights of Ladakh and better road conditions (than amid the Monsoons). As the peak season (June-August) is over, now the rates of the hotel are decreased.
The Ladakh temperature during this time is Maximum 21° C, Minimum 5° C. The climate is cool and wonderful chill begins to look in. It in some cases you can find snows amid late September. Get a chance to enjoy the crystal clear water with glimmering sunbeams falling over the surface and making it sparkle.
Also at this time, the road conditions are much better than the usual. At times there can be snowfall which can cause a roadblock.
Visit some of the best parts of this region like the mesmerizing Pangong Lake and enchanting Tso Moriri and get a chance to learn about the culture and the beauty of this region. Also visit the famous monasteries and the stupas here while you pass through the highest motorable road.