Will Today Be Black Monday? Maybe. (Whatever.)

11/12/2007 02:36 pm ET | Updated May 25, 2011

There's an old saw in the stock market about how the trend on Friday sets the stage for what happens on Monday. The theory is that traders go home a bit giddy or nervous, and after 48 hours of ruminating on their condition, come back to work on Monday absolutely lathered up with fear or greed.

Statistically, this is probably a crock -- most old market saws are -- but then again, they don't call it "Black Monday" for nothing. For example, here's what happened in the days leading up to the biggest market crash in history, October 19th, 1987:

DJIA Closes Mid-October 1987:

Tuesday, October 13: 2,508
Wednesday, October 14: 2,413
Thursday, October 15: 2,355
Friday, October 16: 2,247
Monday, October 19: 1,739

Total Tues-Fri decline: 10%
Total Monday decline: 23%
Total 7-day decline: 31%

And here's what happened last week:

Tuesday, November 6: 13,667
Wednesday, November 7: 13,300
Thursday, November 8: 13,266
Friday, November 9: 13,043
Monday, November 12: ?

Total Tues-Fri decline: 5%
Total Monday change: ?
Total 7-day change: ?

So today could be rough. Or it could be a sucker's trap: a sharp morning panic followed by an afternoon rocket ride toward the moon.

Whatever happens today, ignore all the reassuring commentary about how "fundamentals are solid," "stocks are cheap," "lots of cash is on the sidelines," etc. Part and parcel of a market panic is a parade of experts reassuring everyone that everything is ok. Also part and parcel is that no one knows what will happen next -- so don't pay much attention to the doom-sayers, either.

The unfortunate truth is that the fundamentals aren't "solid" (checked out the financial industry lately) and stocks aren't cheap (record-high profit margins make P/E ratios look artificially low). But these factors will have little effect on short-term moves.

The best survival strategy? Make sure your portfolio is diversified, ignore the forecasts, and hang on to your hat.