The Nepal Himalaya are the highest mountain range on earth, everyone has seen pictures of the snow capped mountains and the extreme heights bring extreme weather. This, coupled with the effects of the monsoon mean that it is important that you choose the right time of year to visit. The monsoon runs from June to early October producing rain at lower altitudes and snow up in the high mountains, these months should be avoided. The monsoon clears the air and in the autumn period from late October to early December the skies are at their clearest producing the best views of the Himalaya. Khumbu nights are below freezing even in November so trekking in the winter months is only for the very hardy!
In spring the skies are generally less clear but this is compensated by the beautiful beautiful spring flowers, in particular the rhododendrons. In the pre-monsoon months of March and April, the skies get increasinly hazy.
My two visits were both taken in late October to November. I would like to see the foothills painted with spring flowers but for me clear skies and predictable (relatively !) weather were the most important factors. In recent years the monsoon has been ending later than predicted and this year (1999) Lobuche and Gorak Shep were virtually impassable in early October.
In the Khumbu (October 27th - November 11th 1999), the weather was marvellous, clear blue skies every day with cold nights, temperatures dropping with height to -8 degrees Centigrade at overnight at Gorak Shep.