This app was mentioned in 3 comments, with an average of 1.33 upvotes
Leave early. Use services like Track the T and the mobile app YourBus MBTA (Android, dunno about iOS or Win) to get an idea of when things are coming, but those aren't absolute. Us Bostonians will be able to give y'all a better idea of what travel will be like closer to PAX but as of right now I'd recommend leaving at least 1 hour, to 1.5 hours of travel time on the MBTA itself before you want to get there.
https://play.google.com/store/apps/details?id=com.rangiworks.transportation.mbta&hl=en gives "real-time bus arrival predictions", has decent reviews
http://openmbta.org/ has realtime bus data
For mobile I use Your Bus MBTA, not sure what the iOS version is but TrackTheT is a browser site app.