Ummm... 4 Octobers out of 80... That's not exactly what I would call a high percentage.. The real stat is that October, historically, has been spotty...lots of ups AND downs...there is no real seasonal trend.. November and December, historically, are very strong for equities...
From 1960, DOW index in October, 61.7% of the same month October is up. The average is up 0.77%, the median is 1.0%, the worst -23.22% in 1987 and the best 10.65% in year 1982. 1987 10 -23.2159 1978 10 -8.4740 1979 10 -7.1570 1997 10 -6.3331 1971 10 -5.4318 1967 10 -5.0633 1977 10 -3.3951 1976 10 -2.5510 1989 10 -1.7729 1992 10 -1.3871 2005 10 -1.2171 1980 10 -0.8505 1995 10 -0.7016 1970 10 -0.6665 1983 10 -0.6431 2004 10 -0.5238 1990 10 -0.4139 1964 10 -0.2616 1960 10 0.0379 1984 10 0.0555 1972 10 0.2360 1981 10 0.3024 1961 10 0.3865 1973 10 1.0010 1994 10 1.6895 1988 10 1.6915 1991 10 1.7346 1968 10 1.7739 1962 10 1.8636 1996 10 2.5026 2001 10 2.5722 2000 10 3.0065 1963 10 3.0623 1965 10 3.2496 1985 10 3.4381 2006 10 3.4391 1993 10 3.5293 1999 10 3.8010 1966 10 4.2430 1969 10 5.2762 1975 10 5.3106 2003 10 5.6718 1986 10 6.2306 1974 10 9.4839 1998 10 9.5565 2002 10 10.6047 1982 10 10.6522 For Nasdaq, since 1985, 59.09% of the months of October is up. The average is up 0.95%. The worst is -27.23 the Oct in 1987. The best is 13.45%. 1987 10 -27.2339 2000 10 -8.2550 1997 10 -5.4624 1990 10 -4.2671 1989 10 -3.6583 2005 10 -1.4588 1988 10 -1.3412 1995 10 -0.7168 1996 10 -0.4409 1994 10 1.7271 1993 10 2.1605 1986 10 2.8799 1991 10 3.0557 1992 10 3.7547 2004 10 4.1200 1985 10 4.3525 1998 10 4.5784 2006 10 4.7945 1999 10 8.0210 2003 10 8.1295 2001 10 12.7702 2002 10 13.4541
It's because october has been some of the more influential drops. That's why it's thought of this way.
I've heard it referred to as the "bear killer", because bears allegedly end in October. October '87's murderous plunge ended exactly one week later, because at that time trades used to settle in 5 instead of the present 3 days. Came settlement day, the market made what proved to be its final bottom, at a point slightly higher than the price it hit on crash day. Basically, they shot the traders who were too far gone to save, buried the bodies, and proceeded to ramp it up from there, never looking back.