from the T Alerts page, and also posted signs:
Due to the Floating Slab Project, buses will replace Red Line trains between Alewife and Harvard Square Stations in both directions from start to end of service beginning Saturday, March 28, 2015, through Sunday, March 29, 2015. Regular Red Line train service will resume at the start of service on Monday, March 30. All shuttle bus stops are accessible for persons with disabilities.
Due to the Floating Slab Project, buses will replace Red Line trains between Alewife and Harvard Square Stations in both directions from start to end of service beginning Saturday, March 28, 2015, through Sunday, March 29, 2015. Regular Red Line train service will resume at the start of service on Monday, March 30. All shuttle bus stops are accessible for persons with disabilities.