In finance, Black Monday refers to Monday, October 19, 1987, when stock markets around the world crashed. The Dow Jones Industrial Average (DJIA) fell 22.61% that day.
Fig. 1. DJIA over the period Jun-1987 to Jan-1988.
Now, in crypto, a price drop of 22% is called 'regular' Monday.
Fig. 2. Bitcoin crashes, Apr-2013 to present.
The take home message is: crypto markets are extremely volatile. If you experience a big drop in price, don't panic sell. The price drop become your loss only after you sell.