Here is one time when the “straight answer” is not going to be clearly “straight.” As stated in the question, Lent does begin on Ash Wednesday and is a special forty-day preparation for the celebration of Easter. Also as stated in the question, the “forty-day” calculation begins with Ash Wednesday, excludes the Sundays of Lent, and ends on Holy Saturday.
The forty-day period of Lent has a long standing tradition in our Church, especially after the legalization of Christianity in AD 313. The Council of Nicea (325), in its disciplinary Canons, noted that two provincial synods should be held each year, “one before the forty days of Lent.” St. Cyril of Alexandria (d. 444) in his series of “Festal Letters” also noted the practices and duration of Lent, emphasizing the forty-day period of fasting. Finally, Pope St. Leo (d. 461) preached that the faithful must “fulfill with their fasts the Apostolic institution of the forty days,” again noting the apostolic origins of Lent. One can safely conclude that by the end of the fourth century, the forty-day period of Easter preparation known as Lent existed, and this period ended at Easter.
Constitution on the Sacred Liturgy of Vatican Council II stated, “The two elements which are especially characteristic of Lent– the recalling of baptism or the preparation for it, and penance– should be given greater emphasis in the liturgy and in liturgical catechesis. It is by means of them that the Church prepares the faithful for the celebration of Easter, while they hear God’s word more frequently and devote more time to prayer” (#109). The Council emphasized, “But the paschal fast must be kept sacred. It should be celebrated everywhere on Good Friday, and where possible should be prolonged throughout Holy Saturday so that the faithful may attain the joys of the Sunday of the resurrection with uplifted and responsive minds (#110). This instruction seems to indicate that the Lenten preparation period of prayer, fasting, and penance continues up through the first Mass of Easter, the Easter Vigil.
However with the liturgical renewal initiated by Vatican Council II, the celebration of the Holy Triduum– Holy Thursday, Good Friday, and Easter– was also examined. Keep in mind that Pope Pius XII actually began this exercise and in 1951 restored the Easter Vigil to its proper place. The individual liturgies of Holy Thursday, Good Friday, and the Easter Vigil are not seen simply as marking isolated events, but really they are seen together as one saving mystery. For this reason, the Mass of the Lord’s Supper on Holy Thursday does not end with a final blessing; rather, that blessing is given at the conclusion of the Easter Vigil. In his beautiful encyclical Ecclesia de Eucharistia, our beloved late Pope John Paul II wrote, “By the gift of the Holy Spirit at Pentecost, the Church was born and set out upon the pathways of the world, yet a decisive moment in her taking shape was certainly the institution of the Eucharist in the Upper room. Her foundation and wellspring is the whole Triduum paschale, but this is as it were gathered up, foreshadowed and ‘concentrated’ for ever in the gift of the Eucharist. In this gift Jesus Christ entrusted to His church the perennial making-present of the pascal mystery. With it He brought about a mysterious ‘oneness in time’ between the Triduum and the passage of the centuries” (#5). One could make the argument, therefore, that Lent ends with the celebration of the Mass of the Lord’s Supper on Holy Thursday, the beginning of the Triduum; however, one would also be left with a less than forty-day Lent, which contradicts long-standing tradition.
So where does that leave us? Perhaps, here is where tradition carries the greatest weight. As stated above, the Second Vatican Council reminded us to keep the paschal fast throughout Lent until the Easter Vigil, the first Mass of Easter. Nevertheless, we must also celebrate the Triduum really as one saving event which allows us to live in the ever-present reality of our Lord’s last supper, passion, death, and resurrection. The Triduum is an even more intensive time of preparation for Easter, and brings Lent to its climax.