The Head of the Charles Regatta is underway in Boston and Cambridge.

Nearly 12,000 athletes from around the world and more than 325,000 spectators are expected to descend on the three-mile course along the Charles River for the multi-day rowing competition.

There were concerns about a toxic algae bloom in the water ahead of the event, but health officials said Thursday that the course area is no longer under an advisory.

Here's what to know about the world-famous regatta.

When is the Head of the Charles?

The Head of the Charles begins Friday with races featuring rowers aged 50+. Events on Friday start at 7:45 a.m. and go through about 11 a.m.

Saturday and Sunday feature full days of races from 7:45 a.m. through about 4 p.m. or later, with the championship races happening later

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