The Hebrew Calendar
The luach ivri - the Hebrew calendar - is calibrated along lunar cycles while remaining anchored to the solar year. The annual cycle of twelve lunar months contains 354 (or 355) days. The solar year based on the annual revolution of the earth around the sun equals 365 days. A 12 day discrepancy exists. The lunar cycle, therefore, drifts from the solar year. But the observance of the festivals in their respective seasons is paramount, and therefore, the lunar year must be brought in line with the solar year. We learn in Exodus that the flight from Egypt occurred during the ripening of the barley:
"And the flax and the barley were smitten; for the barley was in the ear, and the flax was in bloom."
Exodus 9:31
This places Nisan, the month of Pesach - passover - the first month of the year in the spring, after the vernal equinox. In order too keep the lunar year from straying from season, an extra month is counted, seven times in every nineteen years. This is known as a leap year, a year comprising of thirteen months. The thirteenth month is always a second Adar, the month which precedes Nisan - the month of Pesach. In this fashion, Israel navigates through time using the two great lights as its bearings: "And G-d made the two great lights: the greater light to rule the day, and the lesser light to rule the night..." (Genesis 1:16)
"This month shall be unto you the beginning of months; it shall be the first month of the year to you."
Exodus 12:2
The verse refers to the month of Nisan, and this is the first month of the year. Yet, the Hebrew calendar observes four new year's, of which the first of Nisan is but one. Nisan marks the new year regarding the observance of the three pilgrimage festivals, as well as the reigns of kings, whose years were tallied accordingly. Five months later, the first of Elul marks the new year regarding the tithing of animals designated to be brought to the Holy Temple as offerings. One and a half months prior to Nisan, (or two and a half months if a leap year), the fifteenth of Shvat, is the new year regarding the tithing of fruit trees.
"And in the seventh month, on the first day of the month, you shall have a holy convocation: you shall do no manner of servile work; it is a day of blowing the horn unto you."
Exodus 29:1
The first day of the seventh month, Tishrei, is the new year regarding the shmitta, (sabbatical), year, and the the yovel, (jubilee year). It is the month of the creation: our sages teach us that the first day of Tishrei is the sixth day of creation, the day of the creation of man. Therefore, the first day of Tishrei is a day of standing before our Creator and recognizing His sovereignty over all His creation. This recognition demands introspection and judgment. This is Rosh Hashana, the first two days of the month of Tishrei.
The Hebrew year is reckoned from the month of Tishrei. The current year is the year 5767. The reckoning dates back 5767 years ago to the creation of Adam - the first man.
"This month shall be unto you the beginning of months; it shall be the first month of the year to you."
Exodus 12:2
The commandment to declare the new moon and establish its appearance for all the children of Israel was the first commandment received by the Israelites, even before they had emerged from their bondage in Egypt, Establishing the new moon was of such great import for the entire nation of Israel, that it became a matter for the Great Sanhedrin - the highest court in the land. Two witnesses who had seen the appearance of the new moon were required to testify before the Great Sanhedrin, which convened in the Chamber of Hewn Stone, which was located on the northern wall of the Inner Courtyard of the Holy Temple. There they would be questioned and cross examined to verify their fitness as witnesses, and the truth of their words. Only when this had been done to the satisfaction of the sages of the Great Sanhedrin, would the new moon - Rosh Chodesh - be declared. Messengers would be sent forthwith to inform communities of Israel as well as the far flung villages of the diaspora. Special offerings were brought to the altar of the Holy Temple on Rosh Chodesh.
Time was calibrated differently in the Holy Temple. Each day consisted of twenty four hours, but these were twenty four relative hours. This was effected as follows: each day was divided into two periods: the daylight hours were marked from the rising of the sun in the eastern sky to the setting of the sun in the western sky. The nightimes hours were from sunset to sunrise. Each of the two periods would be divided into twelve equal parts. For example, if the daylight hours of a particular day equal eight, then each daylight relative hour will equal forty five minutes. If the daylight hours equal 16, then each relative hour will equal 75 minutes. Therefore, midnight and noon are not fixed hours, but are relative to the the length of the sunlight hours. The daily tasks of the Priests, including the times for the morning and evening Tamid, (continual) offerings, were all scheduled according to the reckoning of the relative hours. (This system of calibrating hours is still used today for fixing the appropriate daily prayer times.)
The days of the week do not have names, but are known simply as First day, Second day, etc.
Finally, one day remains independent of both lunar and solar calculations. This is the Shabbat, the seventh day on which G-d rested after completing His creation:
"And the heaven and the earth were finished, and all the host of them. And on the seventh day G-d finished His work which He had made; and He rested on the seventh day from all His work which He had made. And G-d blessed the seventh day, and hallowed it; because that in it He rested from all His work which G-d in creating had made."
Genesis 2:1-3
Shabbat is the sanctification of time, and stands apart from the other days of the week. This concept, as well as the special relationship that exists between Israel and the Shabbat, is expressed in the following midrashic description:
"Rabbi Shim'on Bar Yochai taught: Shabbat said before the Holy One: 'Master of the universe, every day was given a partner, [Sunday has Monday; Tuesday has Wednesday: and Thursday has Friday], but You did not give me one.' The Holy One answered: Your partner is the community of Israel. And when Israel stands on Mount Sinai, the Holy One will say "Remember the Sabbath to hallow it" (Exodus 20:8) "Remember" I said to Shabbat that the community of Israel is your partner." (Midrash Bereshith Rabba Parsha 11)