Northeast can be visited anytime of the year due to it's pleasant weather and subtropical climate. However as most Northeast states are mainly accessible by road you may need to plan ample amount of days for a hassle free trip🙂
All the states are accessible during all the 12 months. However, travelling to Arunachal or Manipur, or Nagaland, while it's raining heavily might get a little hectic. Meghalaya, Assam, And the rest are very comfortable during all the seasons. however, Assam would be better during winters (keeping in mind the harsh and humid climate during summer!). Sikkim during February end to April would be amazing as well. Hope you have happy travelling the NE India.