The template of 1929 and 1987 is coming into focus in recent trading days.
Let’s start with the 1929 crash. The Dow Jones Industrial Average began the week of October 5 with a slight gain, and then the rest of the week (October 6-9) were all down days as well. This first week down brought stocks to new intermediate lows that had held for several months. The next week, the DJIA opened up down on Monday, October 12 and traded down each day of the week. Then, bang, on October 19, 1929 we experienced a 20% waterfall decline, which carried into Tuesday. The whole 11-day episode erased 28.94% from the DJIA.
Turning to 1987, we can see some similar patterns. The week of October 14 began with a slight up day and then the rest of week was down. The following Monday, October 21 the DJIA started the week down and was down every day that week. Monday, October 21, 1987 took out the intermediate low that had been place for a few months. Then, on Monday, October 28, 1987, the DJIA fell almost 15% in a single session. That was followed by another 10.5% decline on Tuesday, October 29, 1987. Over the course of 10 days, the DJIA lost 34.41%.