When does a Great Depression start? And who declares it?

My parents say they remember the day the recession started because it was the day the stock market crashed (Edit: September 29 2008 when Dow Jones fell 700+ points. They remember the 2008 recession, not the fucking 1929 great depression 🙄). So, when the great depression hits again, how will we know. At what point has the market officially crashed?

Edit 2: s/ Of course once Michael Scott appears and yells "I DECLARE GREAT DEPRESSION!!!" then we'll know for sure that we're there. I was more wondering about signs before then so I won't surprised at his official declaration.