Which is the most profitable day of the year to invest on?

We’ve all heard the old stock market adages: "Sell in May and go away" or warnings of how September is one of the weakest months for many stock markets. But if you are investing for the long-term, do such short-term stock market moves really impact your returns over the long run? Taking that thought one stage further, is there a day of the year on which it is statistically more profitable to start investing? What about the worst day?

To find the answers to these questions, I analysed the performance of the FTSE 100 over every possible five-year timeframe since its inception in December 1983. I analysed over 10,000 data points to calculate what happened if you had invested in the FTSE 100 on each day of the year and sold out five years later (for example, investing on 1st January 1984 and selling on 1st January 1989).