I did a fun little analysis on this query, and the conclusion … the 24th of each month, but not on a Friday.
Of course, this is nothing more than just a pseudo-science that presupposes a majority of stock dividends and bond interest payments are usually paid on the 1st, or the last day, of each month. Some of these payments are reinvested into the same asset, making prices higher. Of course, there’s also the bi-weekly payroll cycle, when most employees make their retirement plan deposits on the 1st and the 15th of the month. So, you definitely do not want put money into the market immediately after these “deposit-active” days.
Monthly stock options expire on the 3rd Friday of each month, causing larger-than-usual price swings, with higher-than-average volume, in asset prices. Most options traders do not like to hold onto shares of stock at expiration, with the risk of volatile weekend price gaps. So, they tend to dump their shares immediately, after trading hours, on the third Friday of the month. If shares are not traded after hours, option traders would purchase put options on the respective stock, which would allow them to sell their shares, at a set and known price, in the following Monday morning. This phenomenon increases the chance of lowering stock prices in periods immediately following options expiration dates.
Guess which day of the month when the two above ideas coincide most often? The 24th. The last day stock options can expire is on a 21st. This means the 1st of the month will fall on a Saturday, leaving 21 days until the third Friday, when options to expire. (The earliest day stock options can expire is on a 15th… meaning the 1st of the month will fall on a Friday, leaving just 15 days to reach the third Friday of the month. You can check all this with a calendar.) The 24th also does not fall into a “deposit-active” day. When these two events coincide, we are theoretically left with the lowest asset prices in the month…which is the best time to Dollar-Cost Average your investments.