Autumn and spring is the trekking season in Nepal. Even October and November is the best tine to do trek in Nepal in autumn. These two months of the dry season when is still comfortably warm as well as clear sky.
December, January and February are still good months for trekking, but it can be bitterly cold at high altitudes and high trekking passes can be closed because heavy snowing.March, April and May a bit warm but also a good time.