Saturday is the Sabbath Day
The Sabbath (or Shabbat as it is called in Hebrew) comes from the root Shin-Beit-Tav, meaning to cease, to end, or to rest. This weekly day of rest has no parallel in any other ancient civilizations. Leisure was for the wealthy and ruling-class only, never for the laboring or the servant. Shabbat involves two interrelated commandments: to remember (zakhor) the Sabbath (Exodus 20:8) and to observe (shamor) the Sabbath (Deuteronomy 5:12).
Nisan-years is an ancient calendar system used around Mesopotamia and is often called the Jewish Religious Calendar. Its beginning was from prehistorical era. Ever since Mesopotamia had historical writings, its calendar used the Nisan-years. It seems that Israel learned of the Nisan-years calendar system from Abraham, who came out of Ur of Chaldees, and so Israel used it consistently through the time of Moses and David.
Nisan-years belongs to the Lunisolar calendar system, in which the lunar years and the solar years are synchronized by adding in an intercalary month in a leap year.
The way to determine whether or not an intercalary month should be added was by agricultural observation in Israel. If by the end of a Nisan-year, barley did not grow spikes, the season is too cold, and an intercalary month should be added so that barley can be harvested in the month of Nisan.
Three ancient civilizations (Babylonia, China, and Israel) knew that by adding in seven intercalary months in nineteen years (called a Small Mahzor in the Jewish Talmudic Calendar), the solar and the lunar years are basically synchronized . Since a tropical year is 365.2422 days, and a synodic month is averaged 29.53059 days, in nineteen years the solar and the lunar calendars will only differ by about two hours.
Nisan-years begins from the Spring season. Technically, its New Years Day is the day after the New Moon closest to the Spring Equinox, when the day and the night is of equal length, set at March 21 in today's Gregorian Calendar. It begins the first month, named Nisanu. The month of Nisan is important since it begins the Spring Feasts of Israel.
Originally the Hebrew calendar was used by Jews for all daily purposes, but following the conquest of Jerusalem by Rome in 63 BCE , Jews began additionally following the Roman calendar, which was decreed by the Roman Emperor in 45 BCE, for civic matters such as the payment of taxes and dealings with government officials.
The first Roman calendar had ten months, starting with March and ending with December, followed by a "winter period" of about 2 months that wasn't reflected on their calendar. This is why the names of the months don't match up with their numbers - October is our 10th Month, even though "octo-" means eight. Originally it was the 8th month. December used to be the 10th month - "deca-" meaning ten. This calendar utilized a number of blank days or "filler" months when necessary to stretch the year to its proper 365 days. This calendar ran on a four-year cycle and had far too many blanks, so it was easily abused for political purposes. For instance, to promote the length of their terms certain rulers added huge amounts of time to their calendar years.
By the time Roman emperor Julius Caesar (100–44 B.C.) took power, the calendar year was so exaggerated that January fell in autumn. Caesar undertook a revision of the calendar. Called the Julian calendar, it consisted of a year made up of 365 days, with one day added every fourth year (called leap year) to compensate for the fact that the solar year is actually 365.25 days. It had twelve months, each having either thirty or thirty-one days except for February, which had twenty-eight.
The days of the week from this Roman period were named after the seven planets of classical astronomy and were numbered the same as the Jewish Religious Calendar, beginning with Sunday as the first day of the week. Saturday is the only day of the week that even today retains its Roman origin in English, named after the pagan god Saturn. The names of the Roman days of the week were:
Sunday - Solis (sun), Monday - Lunae (moon), Tuesday - Martis (mars), Wednesday - Mercurii (Mercury), Thursday - Jovis (Jupiter), Friday - Venelis (Venus), Saturday - Saturni (Saturn).
In later years the Anglo-Saxon's had a strong influence on today's English names for the days of the week. Solis became Sunne, the sun. Lunae became Mona, the moon. Martis became Tiu, the Anglo-saxon god of war, the equivalent of Mars. Mercurii became Woden, the cunning god, the equivalent of Mercury. Jovis became Thor, the Norse god of thunder, the equivalent of Jupiter. Venelis became Freya, the Norse goddess of love and fertility, the equivalent of Venus. And Saturni became Saeter, the Norse god of agriculture and the consort of Ops, the equivalent of Saturn.
Historically, the Hebrew Sabbath began a few minutes before sunrise on Friday evening and ended when the first three stars appeared in the night sky on Saturday evening (Neh. 13:19, Lev. 23:32). The Sabbath is the only ritual instituted in the Ten Commandments given to Moses. It is a sign of respect for the day during which God rested after having completed the Creation in six days, and for God's deliverance from the Egyptians who demanded a seven-day work week from their Israelite slaves. (It can be shown that in 1446 B.C., recognized by scholars as the date of Exodus, Nisan 17 happened to be a Sunday.)
By the 2nd century A.D., the observance of a corporate day of worship on the first day of the week (Solis or Sunday) had become commonplace instead of Saturday because the conquering Roman soldiers demanded that Israel's stores and taverns stay open on Saturday's. The Romans day of rest was on Sunday which was when they paid homage to their sun gods and spent time with their family, so they insisted the Israeli's stay open on Saturday so that their wives could shop during the day and they themselves could get something to drink in the evening. This is attested in the writings of second century writers such as Justin Martyr, Irenaeus, Tertullian, Origen, and Clement of Alexandria.
In 325 A.D., in an attempt to stifle the discord that was happening between the Christians and the pagan Romans,the Roman Emperor Constantine consolidated Christianity with the pagan beliefs the Romans had. One way of doing this was to officially move the Sabbath to Sunday so that the Roman sun gods could be incorporated into Christian worship, whether or not the Christians wanted this to be a part of their worship service. Under the threat of severe punishment or even death, many Christian Elder's allowed this to happen (Ezekiel 8:16-18). They even quit calling it the Sabbath and started calling it the "Lord's Day".
This became so commonplace and stayed that way for so many centuries that Christians today believe that the Sabbath should be on Sunday and become upset if you tell them that, according to the Bible and historical writings, they are worshiping on the wrong day.