It’s hard to generalize about the climate of a country ranging in elevation from near sea level to Mount Everest. About the only thing that can be said is that all but a few parts of Nepal are governed by the same monsoon pattern with temperatures varying according to elevation.
Probably half of all tourists visit Nepal in the autumn (October to November), and for good reasons. The weather is clear and dry, and temperatures aren’t too cold in the high country or too hot in the Tarai. With the air washed clean by the monsoon rains, the mountains are at their most visible, making this the most popular time for trekking. Two major festivals also fall during this season.
In winter (December and January), the snow line descends to 2000 and 3000m, and though it never snows in Kathmandu, the "mists of Indra" make the capital feel cold and clammy. Most travelers head down into India, leaving the trekking routes and guest houses fairly quiet - too quiet, sometimes, as many restaurants pare down their menus for the season.
Spring (February to mid-April) brings warmer temperatures, longer days, weddings and more festivals. The rhododendrons are in bloom in the hills towards the end of this period, and in the Tarai the thatch has been cut, making this the best time for viewing wildlife. The one factor that keeps people away is a disappointing haze that obscures the mountains from lower elevations, though it’s usually possible to trek above it.
The pre-monsoon (mid-April to early June) is stifling at lower elevations, and dusty wind squalls are common. Trek high, where the temperatures are more tolerable.
Nepalese welcome the monsoon (June to September), which breaks the enervating monotony of the previous months, and makes the fields come alive with rushing water and green shoots. The rains rinse and renew the land. This can be a fascinating time to visit, when Nepal is at its most Nepali, but there are many drawbacks, mountain views are rare, leeches come out in force along the mid-elevation trekking routes, roads wash out, flights get cancelled.

It is important for everyone to have a passport that is valid for up to five or ten years. Passports will be necessary for exchanging any foreign currency into local currency. Trip members need to exchange foreign currency upon arrival before they travel into remote areas.