This is the monsoon months in Meghalaya. During these months, the state receives heavy rainfall. In fact, Meghalaya has world's two rainiest cities; Mawsynram and Cherrapunjee. You can plan your trip to Meghalaya anytime during the year, but we recommend that you avoid Monsoons. While the ever beautiful state of Meghalaya looks like a lush green land and spreads a different charm during monsoon when the State receives high rainfall, you might not get to visit all the spots as rain might hinder your way. If the objective is only to spend some rejuvenating time in the hills, monsoon could be an ideal time.
Cherrapunjee and Mawsynram are among the wettest places on Earth; most of the rain falls between June and September, creating very impressive waterfalls and carving out some of Asia’s longest caves. September will be good time to go! Must visit the nongriat root bridges as well.
We visited meghalaya and arunachal last year in September third week and it was absolutely amazing weather didn't face very heavy rainfall and could visit all waterfalls including doubledecker bridge,nohkhali falls and krang suri....so I guess Sep 1st should be fine...hope you find this helpful