Most guide books suggest Oct-Dec and Mar-May as the best times to come to Nepal. We also think Jan-Feb is a great time due to discount airfares during this time to Nepal. It is off-peak season and you will find better deals, but still generally nice weather. Unless you are Peak Climbing, you need not strictly adhere to the seasonal advice. The monsoon is a reality in July and August, but some years (as 2009), it just doesn't really come. Nice weather during the day and rain at night. This may be an anomaly, but check with locals or email us here at nepal-visitor.com to find out about conditions.

What to wear
Comfortable clothes are always appropriate, and be sure they are light, easy to carry and easy to wash. Women will want to wear modest clothes, especially when in rural Nepal (tank tops and shorts are NOT acceptable). You will also want to have a sarong (or you can buy a "lungee" here for about 500NPR), for times when you need to change or bathe without much privacy. River sandals like Chacos or Tevas are a really good choice for everyday shoes. You can do almost anything in these types of shoes.
Also, remember, after you are done trekking, you may want to go out for a night of celebration in Kathmandu, so bring a shirt or something a little dressy that makes you feel good. You will be tired of wearing trekking clothes and t-shirts