Massachusetts reported its second virus-related death Saturday night as the state prepared to open hundreds of day care sites for the children of medical professionals, emergency workers and others who are expected to keep working during the COVID-19 pandemic.

The second person in Massachusetts to die from a coronavirus-related illness was a Middlesex County woman in her 50s who had a preexisting condition, a release from the state health department said. On Friday, a Suffolk County man in his 80s was announced as the first person in the state to die from related illness.

Gov. Charlie Baker announced also Saturday that more than 300 sites will open on Monday, with more to follow later. The rest of the state’s child care facilities have been ordered to shut down to help prevent spread of the coronavirus.

“We know that child care is an especially critical piece of emergency service, and that it allows our front-line workers to continue their battle against COVID-19,” Baker said at a Statehouse news conference.

He said the drop-in sites are reserved for those who must report to work, including hospital workers and other “critical service” workers including grocery store employees. Still, he said the sites should be used only “as a last resort.”

The state has also made progress on increasing its capacity to test for the disease, Baker said, and state officials have been in discussions with the U.S. Army Corps of Engineers to identify facilities that can be converted into temporary medical centers.

More than 520 people have tested positive for the coronavirus in Massachusetts.