Buy on Monday, Sell on Friday
Fridays tend to be the most volatile day on the stock market. The reasons are unclear, but market indexes have traditionally proven to be most unstable at the end of the week. Conversely, Mondays have shown to be strong trading days, particularly following bad Fridays.
Any Day of the Week
Not every shaky trading day falls on a Friday. Sometimes the market plummets or skyrockets on a Tuesday or Wednesday. News directly pertaining to a country's financial markets will always have the biggest impact on stocks. Those occurrences can take place at any time, on any day of the week.
On the basis of consistency over the years, Monday has been the best day of the week for stocks, especially if the previous Friday saw big losses. Yet, this is not a flawless system. When determining when to invest in the stock market, it is best to consult an expert first.