Ex-Dividend is a term used to describe a payment made to shareholders, typically by a publicly traded company as a reward for investing in their stock. It is the date at which shareholders become eligible to receive all the dividends declared by the company, as long as they continue holding the stock.

The ex-dividend date marks the beginning of the period between the date of the declaration of dividend by the company, and the "record date". The record date is the date that determines the eligibility of a shareholder to receive the dividend. To receive dividends, the shareholders must purchase the stock before the ex-dividend date, and maintain the shares until the ‘record date’.

When the ex-dividend date passes, the stock of a company trades at a lower price, as the dividend is considered to be an attribute of the stock, so once it is paid the stock value of the company reduces.

The stock price is affected due to the fact that the shareholders who have not received the dividend, benefit from it, and thus the stock should be traded at a lower value. This reduction in the stock price is known as a dividend adjustment, and allows investors who did not receive the dividend, to benefit from the payment.

Ex-dividend dates are important for shareholders, as it is the date the company's board of directors has specified for when dividend rights and allocations are available. Failing to buy a company’s stock before the ex-dividend date means investors must wait until the next dividend payout in order to receive the payment.

The importance of keeping informed of ex-dividend dates is seen when shareholders and investors are buying dividend paying stocks, as it is only those shareholders who hold the stock before the ex-dividend date that are entitled to their payouts.

In addition, investors must check their brokerage accounts and be familiar with their account's rules regarding the holding of stocks and the paying of dividends. It is important to know the brokerage's rules, as different firms have different policies in place, and some don't pay dividends until after the record date.

In summary, understanding the ex-dividend date of a company’s stock is an important factor for any investor looking to invest in dividend yielding stocks, as it determines eligibility for future dividend payments, and has an effect on the stock’s price. It is important to be aware of ex-dividend dates, as well as the rules around dividend payouts, in order to ensure investors make the best financial decisions.