Instantaneous single click perpetual date mechanism

A date mechanism for a timepiece, which includes a movement arranged to actuate, once per day, the pivoting movement of a perpetual lever comprised in said date mechanism, said mechanism comprising a means of driving a perpetual twelve cam, which includes twelve sectors for the successive months, of different radial dimensions according to the duration of each month, and which completes one revolution per year, wherein said mechanism has only a single click and includes a single click finger, hinged to said perpetual lever and arranged to cooperate directly with a toothing comprised in a thirty-one ratchet wheel which pivots integrally about a pivot axis of a first thirty-one snail cam directly or indirectly controlling a date display means, and a second cam determining the position of a countdown mechanism arranged to adjust the duration of cooperation between said click finger and said wheel according to the current month and the current day of the month and to determine each day the number of teeth of said wheel to be actuated, and said countdown mechanism measuring the duration of the current month on said perpetual twelve cam and according to the position of a pin comprised therein, allowing, limiting or preventing cooperation between a beak of said click finger and said thirty-one ratchet wheel.