Upvote:2
Officially megabus says:
Reservations can be made for trips at least 45 days in advance of departure. Due to operational constraints megabus may choose to change this period.
It seems in practice Megabus has opened bookings in seasonal chunks for the past few years.
Megabus opened bookings for the Fall period after Labor Day to before Thanksgiving around July 21st 2017. I'd expect them to do something similar this year but nothing's guaranteed.