The following is an list of the principal feasts and liturgical seasons from the Catechism:

Feast Date Description
Advent The last 4 Sundays before Christmas

The four weeks prior to Christmas, devoted to preparation for the coming of Christ at Christmas.

Christmas 25 December The feast of the Nativity, the birth of Jesus
Epiphany 6 January

The feast that celebrates the manifestation to the world of the newborn Christ as Messiah, Son of God, and Saviour of the world. The feast of Epiphany celebrates the adoration of Jesus by the wise men (magi) from the east, together with his baptism in the Jordan and the wedding feast of Cana in Galilee.

Annunciation 25 March

The visit of the angel Gabriel to the virgin Mary to inform her that she was to be the mother of the Saviour. After giving her consent to God's word, Mary became the mother of Jesus by the power of the Holy Spirit.

If Jesus were born on 25 December, then His conception would have been nine months earlier, on about 25 March. That is when the angel Gabriel would have announced Jesus’ birth to Mary. Thus 25 March is known in the historic church as The Annunciation.

Holy Week sometime between 15 March and 18 April The week preceding Easter, beginning with Palm (Passion) Sunday. It marks the Church's annual celebration of the events of Christ's passion, death, and resurrection, culminating in the Paschal Mystery.
Easter sometime between 22 March and 25 April

The greatest and oldest Christian feast, which celebrates Christ's Resurrection from the dead. Easter is the "feast of feasts", the solemnity of solemnities, the "Great Sunday". Christians prepare for it during Lent and Holy Week, and catechumens usually receive the Sacraments of Christian Initiation (Baptism, Confirmation, Eucharist) at the Easter Vigil.

The Council of Nice decreed in 325 A.D. that "Easter was to fall upon the first Sunday after the first full moon on or after the Vernal Equinox."  The Vernal Equinox is when there is a 12 hour day - approx. March 21st.

Ascension Sometime between 1 May and 4 June The entry of Jesus' humanity into divine glory in God's heavenly domain, forty days after His Resurrection.
Pentecost sometime between 11 May and 14 June

The 50th day after the Sabbath of the Passover (Easter in the Christian dispensation), thus it is 50 days after Easter Sunday. At the first Pentecost after the Resurrection and Ascension of Jesus, the Holy Spirit came down and filled the apostles.

Whitsun sometime between 11 May and 14 June

Whitsun (or Whitsuntide) starts with Whit Sunday, which is always the 7th Sunday after Easter Sunday. It is usually combined with the feast of Pentecost.

Whit Sunday commemorates the coming of the Holy Spirit in the form of flames to the disciples (Apostles), as recorded in the New Testament. It is often called Pentecost because when the disciples received the Holy Spirit and began to go out and preach about Jesus it was the Jewish festival of Pentecost.

Whit Sunday is a favourite day for baptism. It is thought that as people baptised are often dressed in white it was probably originally 'White Sunday'

Trinity Sunday sometime between 17 May and 20 June

The first Sunday after Pentecost, instituted to honour the Most Holy Trinity (Father, Son, and Holy Ghost).

Corpus Christi sometime between 21 May and 24 June

This is the Thursday following Trinity Sunday to formally celebrate the institution of the Holy Eucharist.

Assumption 15 August

When the Virgin Mary was taken up body and soul into heavenly glory.

 

Other

Immaculate Conception

The belief that from the first moment of her conception, Mary (by the singular grace of God and by virtue of the merits of Jesus Christ) was preserved immune from original sin.

Feast Days

The annual cycle of liturgical celebrations commemorating the saving mysteries of Christ's life, as a participation in the Paschal Mystery, which is celebrated annually at Easter, the "feast of feasts." Feast days commemorating Mary, the Mother of God, and the saints are also celebrated, providing the faithful with examples of those who have been glorified with Christ.

Holy Days of Obligation

Principal feast days on which, in addition to Sundays, Catholics are obligated by Church law to participate in the Eucharist; a precept of the Church.

Lent

Lent is a forty-day period before Easter. It begins on Ash Wednesday. Sundays are skipped when counting the forty days, because Sundays commemorate the Resurrection.

In the Roman Catholic Church, Lent officially ends at sundown on Holy Thursday, with the beginning of the mass of the Lord’s Supper.

Lent is a season of soul-searching and repentance. It is a season for reflection and taking stock. Lent originated in the very earliest days of the Church as a preparatory time for Easter, when the faithful rededicated themselves and when converts were instructed in the faith and prepared for baptism. By observing the forty days of Lent, the individual Christian imitates Jesus’ withdrawal into the wilderness for forty days. Lent has been observed in the church since apostolic times.

In many countries, the last day before Lent (called Mardi Gras, Shrove Tuesday, Carnival, or Fasching) has become a last fling before the solemnity of Lent. For centuries, it was customary to fast by abstaining from meat during Lent, which is why some people call the festival Carnival, which is Latin for farewell to meat.