This according to the MBTA web site:
Orange Line Advisories
DIVERSION: Beginning Saturday, May 3, 2008 at the start of service through the conclusion of service on Sunday, May 4, 2008, a diversion of service will take place between Oak Grove and Haymarket. Substitute bus service will replace regular train service between Oak Grove and Haymarket. Customers desiring service to North Station can remain on the train at Haymarket. Commuter Rail service will stop and Oak Grove and Malden while this diversion is in effect. Please allow extra time for your commute.
Plan your travel accordingly