September to November is best trekking months in Nepal. If you are planning trekking in Nepal in September, it is one of the best months to travel Nepal. In Nepal the monsoon ends around last week of September. It is nice to see clouds giving way. The mountains look perfectly beautiful. You will see vast blue sky from the mountain tops which can be most moving experience.
The Nepali festival teej mostly falls on this month of the year. It will be nice if you can schedule your Nepal visit during this time. In teej festival Nepali women perform dances. In teej you will observe women working hard to make lord Shiva happy for long life, peace and prosperity of husband if married and find the perfect one if are heifer.
You will avoid the crowds when you go trek in early September in Nepal. As you come in transition between monsoon and autumn, you will see clear views and clouds covered peaks simultaneously. The mountain summits for me are beautiful partially covered by clouds than whole naked.
It is popular time to travel Tibet and Mustang region of Nepal in September. And September is my birth month. If you happen to be here in my birthday, there will be a special party (ha-ha). September is nice month also to join Nepal volunteer programs.