The ceremony is usually held outside, they decided they couldn't do that because of the smoke in the air -- so they moved the ceremony indoors. They didn't invite the public in because of COVID-19.
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They rang the bell at 6:59 a.m. the time the South Tower of the World Trade Center collapsed. They will also held a moment of silence and will read the names of the 343 firefighters who died.
The San Francisco Fire Department has done this every year since the attacks happened in 2001 - vowing to never forget.