A calendar mechanism including date-discs, a date program unit, a drive unit that drives the date program unit and a drive member for actuating the drive unit. The date-discs include a first lower units disc and a second upper units disc, and a third lower tens disc and a fourth upper tens disc. The date program unit includes date program wheel sets for actuating the date-discs and arranged to mesh selectively with the date-discs so that the last day of the month is displayed with the third lower tens disc and the first upper units disc, and the first day of the following month is displayed with the fourth upper tens disc and the second upper units disc, the change of date between two consecutive months occurring via a single jump of the fourth upper tens disc and of the second upper units disc, regardless of the month. The mechanism may include a month wheel set and a leap-year adjustment mechanism arranged to cooperate with the drive unit, the drive unit being a month program wheel.