An interesting little table, courtesy of
www.professionalgambler.com
Month over under under %
September 399 461 52.60 %
October 418 473 53.09 %
November 446 491 52.40 %
December 401 370 47.99 %
The point being made here is that bettors react too strongly to the cold winter weather, expecting most games to come in under the total. Oddsmakers account for this bias, and December wagers witness a switch in value from the under to the over.
These numbers are taken from a 16 year period, between 1983 and 1999.
Also, FYI, professionalgambler.com = above average literature, below average picks