When a stock's dividend and/or profits are automatically reinvested in more fund shares, it is known as the ex-dividend date. This is an important concept to understand in the world of finance. It signifies the date on which an investor's shares will increase without any additional action on their part. This date is crucial for investors to keep track of, as it affects the value of their investment. So, it's essential to stay informed about the ex-dividend date to make informed financial decisions.