As the mercury mercilessly marched toward 100 degrees mid-week, Mayor Marc Laredo’s administration announced a bit of relief: Crystal Lake, which had been closed for more than a week due to cyanobacteria (blue-green algae) was to open Thursday morning at 9:30 a.m.

“A huge thank you to Parks & Recreation Deputy Commissioner Mark Kelly who yesterday drove to the testing lab in Connecticut to pick up testing materials, and to Commissioner Marc Welch who delivered the samples to the lab today to expedite this process,” Laredo wrote in an email update Wednesday evening.

Chief Operating Officer Josh Morse was all over social media Thursday spreading the word that lab tests showed the water was safe and that the city would waive Crystal Lake Beach pass fees through the weekend.

The reopening comes just in time for a heatwave and a holiday weekend. The Gath Pool complex is going to be closed all weekend, because there’s a fireworks display scheduled at Albemarle Field on Saturday.

If getting wet isn’t your thing, you can stay cool at the Cooper Center. Laredo announced it would be open Friday from 7 a.m. to 4 p.m. and Saturday from 9 a.m. to 4 p.m., in addition to regular Wednesday and Thursday hours of 7 a.m. to 8 p.m.

This time of year, algae blooms become more common, especially if there hasn’t been much rain. And Massachusetts has been in a drought for several months.

Crystal Lake was also closed for cyanobacteria last August, making this the second closing for algae within a year.