Charlie,
I'm not sure if they will have it running by then, but even if they do, the initial plan is for only 1 RT a day. This is due to the lack of a layover facility in Brunswick, which while under construction will not be ready by the time everything else is ready to go. So the trains must return each night to Portland. This could make it more difficult to take a ride.
Initial service will be two round trips per day from Portland, one through round trip per day from Boston. The start date and the schedule specifics will be announced this summer.
Downeaster Expansion
That article said 2 R/Ts from Boston and 1 from Portland.
I believe that the three round trips is once the project is completed, including the Brunswick train storage shed. I'm not sure that storage shed is under construction, or that a location has even been determined. That facility has been the subject of some rancor in Brunswick.
It is possible that they will run two Brunswick - Boston round trips, with the train dead-headed up from POR in the morning, and back to POR in the evening. It seems like a costly way to operate, but they might do that to increase the usefulness of the extension until it is possible to store trains at Brunswick.