The best time to visit Assam is October to mid March and for Meghalaya best time is October to May but March April May temperature competitively hot i.e. above 30 C. You can travel during any of below month to enjoy the tribal festival.
Khasi Festival - Celebrated in April
Jayantia Festival - 3 days in July
Garo Festival - Celebrated in November
Autumn Festival - Celebrated in October