Boston Red Sox mascot, Wally the Green Monster, gestures while dressed in a medical white coat outside Fenway Park, Monday Feb. 1, 2021, in Boston. Credit: Elise Amendola / AP

BOSTON — The Boston Celtics and Bruins will be able to play in front of a limited number of fans starting March 22, and the Red Sox could host more than 4,500 fans at Fenway Park on opening day.

Gov. Charlie Baker announced Thursday that Massachusetts was moving to Phase IV of its coronavirus reopening plan, which will allow large indoor and outdoor arenas and ballparks to admit up to 12 percent of capacity.

“Opening day is in our near future,” Lt. Gov. Karen Polito said at a news conference in Salem.

The Bruins and Celtics shut down their 2019-20 seasons last March when the pandemic reached the U.S. and finished up on the road in “bubble” environments established by their leagues. Their 2020-21 seasons began this winter in home arenas but without fans.

The Bruins were scheduled to play in front of fans for the first time on Friday night, when they will play the New York Rangers in Madison Square Garden in front of 10 percent capacity, or about 1,800 fans.

“That part of it, yes, is a positive,” Bruins coach Bruce Cassidy said Thursday. “And I would like it to happen all over, if possible.”

Gillette Stadium, the home of the New England Patriots, and Fenway Park have been used as mass vaccination sites. Baker said he did not know what would happen with that capacity when the venues return to their regular purposes.