We do have couple of the best season to visit Nepal throughout the year such as in Spring:- March, April, May and in Autumn:- September, October, November. If you prefer high altitude adventure trip with best possible clarity then you should plan either Spring or Autumn. However you can also visit Jun, July, August and December, January, February. If you prefer less crowed and short trekking/hiking in low elevation then you should trek in off season too. Specially if you choose winter like December, January, February then the clarity will so high ever. The only thing is it might be more cold but as long as you have better equipment then it won’t be problem. Jun, July and August are known as rainy or summer season in Nepal. This is the best time to exploring lush.