Holy Days of Obligation
January 1, the Solemnity of Mary, Mother of God
The Solemnity of Mary Mother of God falls exactly one week after Christmas, the end of the octave of Christmas. It is fitting to honor Mary as Mother of Jesus, following the birth of Christ.
When Catholics celebrate the Solemnity of Mary Mother of God we are not only honoring Mary, who was chosen among all women throughout history to bear God incarnate, but we are also honoring our Lord, who is fully God and fully human.
https://www.americaneedsfatima.org/Our-Blessed-Mother/solemnity-of-mary-mother-of-god.html
Thursday of the Sixth Week of Easter, the Solemnity of the Ascension
The Ascension serves as an ultimate demonstration of Jesus’ power over death, inaugurating his reign over heaven and earth as the Living King. When Jesus ascended into heaven, two significant events in particular happened: Jesus gave his followers instructions on how they were to live, and he left them with a promise. He directed the disciples to spread the message of the Gospel to the ends of the earth, declaring the good news of salvation.
https://www.nationalshrine.org/blog/why-do-we-observe-the-solemnity-of-the-ascension/
August 15, the Solemnity of the Assumption of the Blessed Virgin Mary
On November 1, 1950, Pius XII defined the dogma of the Assumption. Thus he solemnly proclaimed that the belief whereby the Blessed Virgin Mary, at the close of her earthly life, was taken up, body and soul, into the glory of heaven, definitively forms part of the deposit of faith, received from the Apostles. To avoid all that is uncertain the Pope did not state either the manner or the circumstances of time and place in which the Assumption took place — only the fact of the Assumption of Mary, body and soul, into the glory of heaven, is the matter of the definition.
https://www.catholicculture.org/culture/liturgicalyear/calendar/day.cfm?date=2020-08-15
November 1, the Solemnity of All Saints
All Saints' Day is a solemn holy day of the Catholic Church celebrated annually on November 1. The day is dedicated to the saints of the Church, that is, all those who have attained heaven. It should not be confused with All Souls' Day, which is observed on November 2, and is dedicated to those who have died and not yet reached heaven.
December 8, the Solemnity of the Immaculate Conception
Through the centuries the Church has become ever more aware that Mary, "full of grace" through God, was redeemed from the moment of her conception. That is what the dogma of the Immaculate Conception confesses, as Blessed Pope Pius IX proclaimed on December 8, 1854: "The most Blessed Virgin Mary was, from the first moment of her conception, by a singular grace and privilege of almighty God and by virtue of the merits of Jesus Christ, Saviour of the human race, preserved immune from all stain of original sin." — Catechism of the Catholic Church
https://www.catholicculture.org/culture/liturgicalyear/calendar/day.cfm?date=2019-12-09
December 25, the Solemnity of the Nativity of Our Lord Jesus Christ
Today the Church celebrates the Birth of Jesus Christ, the first day in the octave of Christmas. Throughout Advent the Church longed ardently for the coming of our Savior. Today she celebrates His birth with unrestrained joy. "The Word was made flesh and dwelt among us." The Son of God became man to give us a share in that divine life which is eternally His in the Blessed Trinity. Christmas time begins on December 24 with the first Vespers of the feast and ends on the feast of the Baptism of Christ. White vestments reappear in our churches as a sign of joy.
https://www.catholicculture.org/culture/liturgicalyear/calendar/day.cfm?date=2019-12-25