Is It Just an Expression?

Investors incessantly say:
"Sell in May, and then go away,"
And stock market nerds
Who heeded these words
Are 6% richer today.

Via the FT's Alphaville blog, we learn that May 2012 was a very bad month indeed for US equities:

The Dow Jones Industrial Average had its worst month in two years in May. The index fell 6.2 per cent, the largest decline since dropping 7.9 per cent in May 2010 (Wall Street Journal). Closing down 0.23 per cent at 1,310 on Thursday, the S&P 500 had its worst month since last September (Reuters).

A strict observance of the "Sell in May then go away" rule, executed on the first of the month, would therefore have yielded the best results.