The first day when the common stock is subject to a rights offering trades without the rights attached.
Applying "Ex Rights Date" to Securities Exams:
A shareholder’s preemptive right to maintain their percentage ownership in a corporation is done through a rights offering. Every outstanding share of the stock will receive a right when the offering is completed. During the time the stock is trading with the rights attached the stock is said to be trading cum rights. The first day when new buyers of the stock will not receive the right with the stock is known as the ex rights date. The ex rights date is also the first day the stock trades without the rights attached.