The Paschal Triduum is counted the way the ancient world counted, inclusively and from evening: from the Mass of the Lord's Supper on the evening of Maundy Thursday, through Good Friday and Holy Saturday, to evening prayer on Easter Sunday. Three days, but one observance. The liturgies are deliberately continuous: the Thursday Mass has no dismissal, the Friday liturgy has no true beginning, opening in silence where the previous night left off, and Holy Saturday has almost no liturgy at all, the church keeping still at the tomb until the Vigil breaks the silence in the night.
The earliest Christians did not distribute these commemorations across separate days; they kept a single paschal night that held death and resurrection together, and the Triduum as the Roman books now present it is a deliberate return to that unity, ranked above every other observance of the year so that nothing, not even the greatest saint's day, can displace any part of it. The three days also form the hinge of the whole calendar: Lent ends as the Triduum begins, Eastertide begins as it ends, and every movable feast of the year, from Ash Wednesday to Pentecost, is counted forward or back from the Easter at its centre. The atlas keeps the Triduum as its own entry because the season it crowns cannot be understood with its summit missing.