The weather is probably the best guide for deciding when to plan your trip to Nepal. October and November are considered the best times of the year. The monsoon will have just ended, and clear skies with optimal temperature will prevail. The main festivals of Dashain and Tihar (Hindu equivalent of Christmas in terms of festivity) fall during these months. However, this is also the busiest tourist season, and the main tourist centers and trekking trails tend to be crowded with travelers like you. The tourist flow ebbs a little, but not significantly, between the winter months of December and mid-February. It catches up once again between mid-February and mid-April. From mid-June to early October, it's the monsoon, during which time it rains almost everyday and most of the Himalayas are hidden behind the clouds. Check the weather section of this FAQ for more details on weather. In short, plan to visit Nepal between October and May, keeping in mind that October-November and February-March are the best times (but crowded with other travelers).