The ex-dividend date is important for investors because it determines whether they will be eligible to receive the upcoming dividend payment. Investors who purchase shares before the ex-dividend date will be documented as owners of shares on the record date and will be entitled to receive the dividend payment. On the other hand, investors who purchase shares on or after the ex-dividend date won't be recognized as shareowners on the record date and the seller will still be the owner of the record and will receive the dividend payment.