On the declaration date the dividend becomes a legal liability of the company. The press release names three other dates that follow: the ex-dividend-date, the record-date, and the payment-date. Until declaration there is no obligation, which is why a dividend is never guaranteed.
The declaration is the news event. A raise, a freeze, a special-dividend, or a dividend-cut all land here, and for an income-stock the reaction can be larger than the reaction to earnings.
Example: on 12 February the board declares $0.62 payable 15 April to holders of record 20 March, with the stock going ex on 20 March. A holder who buys on 21 March receives nothing for that quarter.
Related: ex-dividend-date, record-date, payment-date, dividend