The ex-dividend date is the first trading day a buyer of the stock no longer receives the declared dividend; the price is expected to open lower by roughly the dividend. For options, it is the date short in-the-money calls face rational early assignment when the dividend exceeds their remaining time value.
The mechanics, the pricing effects, and the assignment comparison are covered in dividends and options. As a calendar item it belongs on the same watch list as earnings dates: both are scheduled events that change a position's statistics mid-trade, and screens that gate on one extend naturally to the other.