In the ancient Christian Church (1st-4th centuries A.D.), the two main seasons of the year revolved around the three greatest festivals of the Faith, namely the Nativity (birth) of Jesus Christ, later called Christmas, in December, and Good Friday and Resurrection Sunday called Easter. Advent was the season leading up to Christmas, and Lent, which meant “spring,” which occurred in the season of spring, led up to Holy Week: Good Friday and Easter. Added to this was the long season of Pentecost, about a 6-month season, which runs from Pentecost Sunday in late spring (about 50 days after Easter) all the way to the start of Advent.
The next article below is from a Roman Catholic Church layman M. Lukowski, an essay he wrote for an adult study course. I have permission to post this on this site. My comments I add are these. It is a good overview of the Christian Church year. Second, “Liturgical” comes from the Gk. word liturgia which means worship. It came to mean a set form of worship, written down which includes rites. Here, the term is used generally synonymously with the Church year.