The Church calendar is also called the "Liturgical Calendar." It has been the traditional practice of civilized peoples to have the cycle of the tunes of the year associated with their religious practices. This has been true of the Jewish calendar, the calendars of the Moslems, which begin with the year I corresponding to the year A.D. 622, as well as the Chinese and Japanese calendars.

The early Church calendar followed the time cycles of twelve lunar months, amounting to 364 days in the year, but this was incorrect and could not be brought into proper reckoning with the Julian Calendar. There arose severe differences in the Church over the timing of feasts. Then in A.D. 325 at the Council of Nicea, it was decided that Easter was to be determined as on the first Sunday following the first full moon of the spring equinox.

There were inaccuracies in the Julian Calendar in calculating the length of the year and this error, amounting to ten days by 1582, was corrected by the Gregorian Calendar under the urging of Pope Gregory the XIII. The accumulated error of ten days was eliminated, and the slight astronomical difference was corrected by the new reckoning of leap-year. Even today this is not entirely corrected since according to the true astronomical year the twelve-month cycle is off by 26 seconds annually.

With this brief background we turn to the Church calendar, which for the Roman Church is an arrangement of a series of liturgical seasons throughout the year and a daily assignment of feasts of the saints or commemorations. There is unity and harmony in this procedural course of integrating each day of the year into a continuing, interrelated cycle of divine worship that takes into account both the daily celebration of the Eucharist and the Liturgy  of the Hours.
Vatican II stressed the importance of the Church calendar and pointed to a new assessment of feasts and complete reorganization.

"Holy Mother Church is conscious that site must celebrate the saving work of her divine Spouse by devoutly recalling it on certain days throughout the course of the Year. Every week, on the day, which she has called the Lord's day, she keeps the memory of His resurrection. In the supreme solemnity of Easter site also makes an annual commemoration of the resurrection, along with the Lord's blessed passion.

"Within the cycle of a year, moreover, she unfolds the whole mystery of Christ, not only from His incarnation and birth until His ascension but also as reflected in the day of Pentecost, and the expectation of a blessed, hoped-for-return of the Lord.

"Recalling thus the in mysteries of redemption, the Church opens to the faithful the riches of her Lord's powers and merits, so that these are in some way made present at all times, and the faithful are enabled to lay hold of them and become filled with saving grace.

"In celebrating this annual cycle of Christ's mysteries, holy Church honors with special love the Blessed Mary, Mother of God, who is joined by an inseparable bond to the saving work of her Son. In her the Church holds up and admires the most excellent fruit of the redemption, and joyfully contemplates, as in a faultless model, that which she herself wholly, desires and hopes to be.

"The Church has also included in the annual cycle, days devoted to the memory of the martyrs and the other saints. Raised up to perfection by the manifold grace of God, and already in possession of eternal salvation, the, sing God's perfect praise in heaven and offer prayers for us. By celebrating the passage of these saints from earth to heaven the Church proclaims the paschal mystery as achieved in the saints who have suffered and been glorified with Christ; she proposes them to the faithful as examples who draw all to the Father through Christ, and through their merits she pleads for God's favors.

"Finally, in the various seasons of the year and according to her traditional discipline, the Church completes the formation of the faithful by means of pious practices for soul arid body, by instruction, prayer, and works of penance and of mercy.”

Accordingly, this most sacred Council has seen fit to decree as follows:
"By an apostolic tradition which took its origin from the very day of Christ's resurrection, the Church celebrates the paschal mystery every eighth day'; with good reason this, then, bears the name of the Lord's day or the day of the Lord. For on this day Christ's faithful should come together into one place so that by hearing the word of God and taking part in the Eucharist, they may call to mind the passion, the resurrection, and the glorification of the Lord Jesus, and may thank God who 'gave us new birth; a birth unto hope which draws its life from the resurrection of Jesus Christ from the dead' (1 Pt. 1:3). Hence the Lord's day is the original feast day, and it should be proposed to the piety of the faithful and taught to them in such a way that it may become in fact a day of joy and of freedom from work. Other celebrations, unless they be truly of overriding importance, must not have precedence over this day, which is the foundation and nucleus of the whole liturgical year.

"The liturgical year is to be revised so that the traditional customs and discipline of the sacred seasons can be preserved or restored to meet the conditions of modern times; their specific character is to be retained so that they duly nourish the piety of the faithful who celebrate the mysteries of' Christian redemption, and above all the paschal mystery" (SC: 102-107).

Thus on May 9, 1969, Pope Paul VI gave his approval for a complete reorganization of the Church Calendar declaring the purpose to be that the faithful (may) communicate in a more intense way, through faith, hope and love in the 'whole mystery of Christ which ... unfolds within the cycle of the year."'

The new calendar was promulgated and went into effect Jan. 1, 1970, with the full implementation made after the work on the liturgical texts was completed. In 1972 the Bishops of the US and many other countries ordered the new calendar into effect, and usage was complete with the introduction of the Sacramentary in 1974. The general norms listed for the Liturgical Calendar are: "The Church celebrates the memory of Christ's saving work on appointed days in the course of the year. Every week the Church celebrates the memorial of the resurrection on Sundays, which is called the Lord's Day. This is also celebrated, together with the passion of Jesus, on the great feast of Easter once a year. Throughout the year the entire mystery of Christ is unfolded, and the birthdays (days of death) of the saints are commemorated." (Exceptions are the Feast of St. John the Baptist who is honored on the day of his birth and Sts. Basil the Great and Gregory Nazianzen, and Sts. Cvril and Methodius who have shared feasts.)

"A liturgical day runs from midnight to midnight, but the observance of Sunday and of solemnities begins with the evening of the preceding day," and thus the celebration of the Eucharist, with fulfillment of the Sunday or holy day obligation, may be on the evening before.

Source; Catholic Encyclopedia, Thomas Nelson Publishing, 1990