The organization of each liturgical year is governed by the Church and ultimately integrated into a liturgical calendar.
The Second Vatican Council brought renewed emphasis to Sunday as a unique liturgical category: "the Lord's day is the original feast day" (Sacrosanctum Concilium, no. 106), and it "must be observed in the universal Church as the primordial holy day of obligation" (Code of Canon Law, canon 1246 §1). Thus, only a limited number of feasts of the Lord or the saints may take the place of the scheduled Sunday celebration.
Saints and other celebrations are distinguished in accordance with the importance assigned to each one: each is a Solemnity, Feast, or Memorial. Sundays and Solemnities begin their celebration on the evening before, Feasts and Memorials are celebrated over the course of one day, and Memorials are either Obligatory or Optional.
Finally, holy days of obligation (also known as feasts of precept) are days when the faithful are obliged to participate at Mass and abstain from unnecessary work or other activities which hinder the suitable relaxation of mind and body. Each Sunday is a holy day of obligation, and six Solemnities are also observed as feasts of precept in the United States. (Source: USCCB)
The Year of Grace, 2021
With Evening Prayer I of the First Sunday of Advent, the new Year of the Church for 2021 begins. It runs from Sunday, November 29, 2020 through November 27, 2021.
Annually, the Office of Worship issues "Precepts for Celebrating the Liturgical Calendar" to assist parishes in forming the liturgical calendar for their particular community or congregation. Other supplements to this document are issued on an as-needed basis, particualrly as relates to the "strong seasons" of Advent, Christmas, Lent and Easter.
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