Hello.. I m from North East and glad that you choosed to travel here.. Well to start with you can't expect to get all the fun in a single visit but m sure once you visit, you will be tempted to visit here again..Now the tourist places-
Meghalaya is famous for its waterfalls,root bridges and caves. Best time for the falls is the monsoon season where it flows in full flow whereas you can't explore the caves during that time. For caving you need to choose drier season say Jan..For Kaziranga in Assam, you need to visit during Oct-Mar coz thats when the safari is at its best to site the Rhinos. For Tawang and Sikkim, best season is April-May as its the time the snow just starts to melt and you will get the experience of snow without getting blocked for any day due to road blockages.For Dzukou valley, trek in the month of Jun-July to get the eye catching green carpet..
So you see there is a lot to explore and the month completely depends upon your choice of the place.. P.S. do visit my photoblogs to view the North East tours..
Northeast has a pleasant weather and good road conditions making it accessible all through the year. But personally I feel anytime between April to October is the best time as nature is at its best then.