At the beginning of the first century bce, the Roman Republic had a lunar calendar. About half-way through the century, Julius Ceasar asked the astrologer (read: early astronomer) Sosigenes to design a new calendrical system. Sosigenes proposed a solar calendar based on 365 and a quarter days, which Julius Ceasar instituted in the year we now designate as 46 bce. To deal with the extra quarter-day per year, an additional day was intercalated into February once every four years (i.e., in every year divisible by 4). This calendar, known as the Julian Calendar, was widely adopted around the Mediterranean world as the Roman Empire expanded.
The Julian Calendar remained in use for over 1500 years without any serious adjustment. But towards the end of that period a small problem became apparent, namely, the solar year is not 365.25 days long, but 365.242199 days. This difference of 11 minutes and 14 second each year led to a difference between the solar year and the calendar year of nearly 1.5 days every two centuries and 7 days every millennium. After 1500 years, the discrepancy was noticed in the non-alignment of the seasons.
Pope Gregory XIII fixed the calendar in 1582, and the calendar is now known as the Gregorian Calendar. He issued a papal bull that made several decrees. First, to fix the descrepancy, he removed ten days from the month of October in 1582. Second, to keep the calendar from going wrong again, he accepted the value of 365.2422 days per solar year. Since the difference amounted to 3.12 days for every four centuries, he decreed that every centennial year not divisible by 400 was not a leap year, even though it was divisible by four. Thus, the years 1700, 1800, and 1900 were not leap years, but the year 2000 will be. Third, he standardized New Year's Day as January 1st. Many countries had observed different days as the New Year; Britain for example had treated Christmas Day, December 25th, as New Years.
Given the divisions between the Catholic Church, the Orthdox Church, and the Protestants, it took a long time for the Gregorian Calendar to be adopted. Most of the Catholic nations changed over within the following century, while most of the Protestant nations did not change until the end of a second century. Most Orthodox and non-Christian countries did not accept the calendar until the late 1800s or the 1900s (although the Orthodox Church still uses the Julian Calendar to determine its annual cyld of festivals and holy days). The Gregorian Calendar is now the standard civil (as opposed to religious) calendar in the world. (For a discussion of Julian and Gregorian Calendars and a list of when different countries made the switch, go here.)
In this calendar, the year 1 is set at the year of Jesus' birth. The years following that date are counted up and are designated as "Anno Domini," "The Year of our Lord (Jesus Christ)" and are abbreviated A.D. The years prior to that date are counted down to year one and are designated "Before Christ," and are abbreviated B.C. As non-Christian countries adopted the Gregorian Calendar, they changed the meaning of the years to "Common Era" (abbreviated C.E.) and "Before Common Era" (B.C.E.), while keeping year one at the same date.
In Christianity, worship consists primarily of two things. First are things that are said, such as prayers, Scripture readings, the singing of Psalms or hymns, and preaching. Second are things that are done, the main act being the observance of the Eucharist (Holy Communion). While prayers and Bible reading may be done either in private or in public, the Eucharist is always celebrated in public. Christianity has always encouraged regular, private devotion, usually on a daily basis. The following discussion, however, is about public worship.
The Eucharist, also known as Holy Communion or Mass, celebrates the sacrifice that Jesus made on the cross to bring salvation to humanity. It is modeled after the Jewish sacrificial rites at the Jerusalem Temple, with Jesus himself as the victim. During the Eucharist, the worshipper is invited eat a piece of bread (sometime unleavened) and drink a small amount of wine (or grape juice). The bread represents Christ's sacrificed body and the wine his blood. In some theological explanations, the bread IS Jesus' body and the wine IS his blood. By eating the god, whether symbolically or literally, the worshipper receives forgiveness for their sins and links themselves to the salvation that Jesus' death and resurrection brought about. Nearly every form of Christianity considers the Eucharist as a sacrament, and has a religious official (such as a priest, minister, pastor, etc.) officiating over this public ceremony.
The importance of daily public worship has varied widely over the centuries. At times, the church held daily services of the Eucharist (i.e., Mass), which lay people and priests alike were expected to attend. Monastic communities, which were designed to give monks time to worship God, established a daily regimum of prayer and praise to God called the Liturgy of the Hours. They would rise early to pray and then pray at regular times through the day, with the last prayer service just before retiring at night. In recent centuries, the Anglican Church has preserved daily public prayer, although it is observed more in the breach than by attendance.
The main weekly worship in Christianity happens on Sunday, a day when all Christians are expected to attend a public worship service. Although at first glance, it might appear that Sunday is simply a borrowing of the Jewish Sabbath, it is actually much more than that. It is actually a "mini" Easter. Since Easter, which celebrates Christ's resurrection and triumph over death, happens on a Sunday, the Christian day of worship was placed on Sunday to continue that celebration throughout the year.
In Catholicism, the Sunday service has been called the Mass (Mass is not limited to Sunday only); through prayer, preaching, Scripture reading, and singing, the Mass' structure leads up to the Eucharist as the climax of the service. A similar type of service is held in the Orthodox Church. Until Vatican II in 1965, this service was conducted in Latin.
In Protestant Churches, the weekly service usually focuses on things that the people and the pastor say: hymn and Psalm singing, preaching, Scripture reading and prayer. Communion is celebrated much less frequently--monthly, or sometimes only once every three months.
Christianity has two annual cycles. First, there is the cycle of holidays that center around key events in Christ's life on earth. This is called the temporal cycle and is celebrated by all three main branches of Christianity. Second, there is the cycle of holidays that commemorate (1) people associated with Jesus, such as Joseph, Mary and John the Baptist, and (2) various saints. This is called the sanctoral cycle and is observed primarily in the Catholic and Orthodox branches; most protestant churches rejected it.
From the earliest Christian centuries, there have essentially been three main holidays celebrating key events of Jesus' life on earth. Originally, they were single days but each has been expanded into a series of observances and commemorations.
Easter was taken over from the Jewish celebration of Passover. That is why it is also called the Pascha, after the Paschal lamb offered at Passover. Although it originally celebrated Christ's resurrection on Easter Sunday, it was expanded into the season of Lent for preparation leading upto a "Holy Week" celebration. The Week began with Palm Sunday (which celebrates Jesus' entry into Jerusalem) the Sunday prior to Easter, Ash Wednesday (which observed his betrayal and trial), Maunday Thursday (which commemorates the Lord's Supper and the agony in Gethsemane), Good Friday (which recalls his crucifixion).
Pentecost also comes from a Jewish festival, that of Pentecost, which comes 50 days after Passover. It commemorated the descent of the Holy Spirit on the first Christians. Ascension Day--the day Jesus ascended into heaven--was attached to it early on, since it was just 40 days after Easter. The 50-day period between Easter and Pentecost also became a time of joy during which Jesus' resurrection and gift of salvation was celebrated. This extended period became known as Pentecost as well, and during certain time periods, actually superceded the day of Pentecost in importance.
The third important religious festival began as Epiphany and had no links to Jewish holidays. Its primary celebration was the manification (incarnation) of God on earth in the form of Jesus. Thus it celebrated Jesus' birth, baptism and his first miracle, that of changing the water into wine at Cana. In the Western Church, the establishment of Christmas overshadowed the rest of Epiphany and shifting the emphasis solely to Jesus' birth (i.e., Nativity). The festival of Epiphany then shifted to a celebration of the arrival of the magi.
These three main feasts have developed into a two-season annual cycle. The first is Advent-Christmas-Epiphany, while the second is Lent-Easter-Pentecost. The importance of the day of Pentecost, especially in Protestant Churches, has become subsumed by the season of Pentecost and its continued celebration of the joy of Easter.
The Sanctoral Cycle is the termed used by the Catholic Church to designate the annual cycle of feasts and memorials to the saints and the people associated with Jesus' activities such as Mary and Joseph. The Orthodox Church uses the term menaion. The earliest saint to whom a feast was celebrated is Polycarp, the Bishop of Smyrna who was martyred in 155. Over the centuries many more saints were added to the cycle. Some would simply be included in the list of saints read on Sundays, while other had feast days commemorating their example and ascension into the heavenly realm added to the calendar. While all Christians were expected to observe these holy days, it became quite impractical. As more saints were added to the calendar, the importance of their days began to overshadow the significance of Sunday. The Catholic Church tried to take control of the adding of new saints by establishing the process of canonization, but this only slowed it down. In many areas, churches, cities and towns, trade guilds, and other groups established a patron saint who they believed looked after their particular interests. Similarly, people began to be given saint's names, with the expectation that that saint would look after the individual. (For last year's calendar of Saints' days, go here.)
The Protestant Reformation rejected the specific saints' days, opting instead for a generalized All Saints Day. At various Church councils, the Catholic Church revised the calendar, setting up hierarchies of importance of various saints. The most recent revision occurred as part of Vatican II, which reduced many saints' days to mere "memorials" rather than feasts.