Pre-monsoon season (April, May and June); it can be quite warm by midday, but expect frost at night with the possibility of a dip towards -15°C or so in cold weather.
Post-monsoon (late September, October and November): about 5°C colder than pre-monsoon.
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.
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.
Looking for a guided trip with an experienced company? We recommend KE Adventure Travel's Everest Basecamp Trek.