In the main Himalayan chain pre-monsoon (April, May and June) it can be quite warm by midday, but expect frost at night on the highest sections of the trek with the possibility of a dip towards -10°C or so in cold weather.
Post-monsoon (late September, October and November) is usually a little colder than pre-monsoon.
In the Karakoram and rain-shadow areas trekking may continue through the summer.
Wind chill rating: Low
Many treks are supported by porters. So although there may be a restriction on the total weight of your gear, it is not so critical. Of course if you are carrying all your own gear, the remarks on lightweight apply fully.
Lower down on the trek it is likely to be a lot warmer than the quoted figures, so it is advisable to go for a full zip on your sleeping bag.