Google Trends search on "Sell in May" yielded some interesting results.
Search on this topic hit an 8-year high in 2007. This would have been a good time to sell most equity markets. US S&P had a double peak in June and October 2007. HSI hit a peak of 30,000+ change in October 2007, up 48% from May 2007. By Aug 2008 HSI was back to end May 2007 levels, and subsequently dropped in Fall of 2008.
Most searchers were from Germany, the US, and the UK; in that order.
Interesting that May 2008 had the lowest number of searches on this topic since records were kept. Investors likely had much more on their mind than the boring Halloween Effect/Sell-in-May strategy.Link to Google Trends