There is no way to have more than a surface understanding of the events in Scripture without a working familiarity with the Hebrew calendar. It forms the backdrop for all the festivals and appointed times. The reason we need a calendar at all is because of three factors, which at present do not precisely line up with one another:
(1) The orbit of the moon around the earth (a month), which at present takes about 29.5 days.
(2) The rotation of the earth on its axis (a day), which takes about 24 hours.
(3) The revolution of the earth around the sun (a year), which requires about 365.24 days.
Most of us are more used to thinking in terms of the Gregorian calendar (a revision of the Julian calendar of Rome made official by Pope Gregory XIII). This calendar has two segments, traditionally labeled B.C. ("before Christ", which counts down to Yahshua's birth) and A.D. (anno domini, or "the year of our master", which counts upward since his birth, though the count is off by a few years). While most nations have adopted the same calendar, many, especially those who do not wish to recognize any authority of the Roman Catholic Church to name the era, use the terms C.E. ("Common Era") for the present era and B.C.E. ("Before Common Era", the equivalent of B.C. in numbering).
The Jewish calendar, in contrast, starts with creation, and the current date is derived by adding up the ages of people mentioned in Scripture and historical records since that time. For example, the Jewish year that overlaps with both 2007 and 2008 C.E. is numbered 5768.
This also shows that the year does not begin at the same time on both calendars. The Jewish year begins not on January 1, but on Rosh haShanah ("head of the year", but called Yom Teruah in Scripture), which falls on a new moon in the early autumn. That is when the numbered year changes. This follows the anniversary of Creation and the calendar that was in effect for the whole world until the time of the Exodus, and continued to be the "civil calendar" for Israel thereafter as well, much as many organizations have a "fiscal year" which begins halfway between January 1 and December 31 or even at other times.
But there is another calendar in Scripture which was given exclusively to Israel. YHWH declared that the month in which He brought Israel out of Egypt would now be the first month for us. (Shemoth/Exodus 12:2) This is six months after Yom Teruah, so that Yom Teruah begins the seventh month on the Hebrew calendar while it remains the first new moon on the civil calendar.
Both calendars continue to be used throughout Scripture. They run simultaneously. We have to look at contextual clues to determine which is being referred to in each particular case. While we use the Gregorian calendar with the world at large, and may have to initially translate from one to the other while we make the transition, we want to get to the point where, for example, if someone says, "When is Hanukkah this year?", the only answer that comes to mind is "On the 25th day of the 9th month, of course! That's when it always starts!"
The reason it "moves around" the Gregorian calendar is because a year is now never exactly twelve months long, and both calendars deal with this in different ways. The Gregorian calendar adds a day every "leap year", for example. The Jewish calendar adds a whole month every few years instead to keep the solar and lunar calendars aligned.
The way dates were determined in Biblical times was according to two major factors:
(1) The sighting of the new moon, which defines a new month. (Witnesses would go before the Sanhedrin and answer questions to determine if what they had seen fit the characteristics of a new moon.) The month begins when the moon is sighted in Israel, not the international date line or wherever in the world it might be first sighted, because "the Torah shall go forth from Tzion." (Mikha 4:2) It then continues westward as the sun sets in each time zone.
(2) The Aviv (which is when ripening barley reaches a stage at which firstfruits can be harvested in two weeks--see photo at right), which determines the first month of the year, in which Passover and this firstfruits offering must occur. Because the Aviv is not always reached by the end of the twelfth month, a thirteenth month must sometimes be added in order to keep the festivals aligned with the proper seasons of the year. The blooming of the almond tree is also noted in Scripture as a sign that the new year is about to begin--a reminder to watch for the Aviv.
The present-day Jewish calendar was developed out of necessity in the 4th century C.E. by Hillel II after there was no longer a large Jewish presence in the Land of Israel and even if someone was there to sight the new moon, it could not be communicated quickly enough to exiles in every location around the world, especially on days such as Rosh haShanah (Yom Teruah), when the festival begins right on the new moon. So a calculated calendar was devised which standardized the dates for many years to come. It determined that within a cycle of 19 years, a thirteenth month is added during the third, sixth, eighth, eleventh, fourteenth, seventeenth, and nineteenth years.
The extra month is added between the eleventh and twelfth months (fifth and sixth on the civil calendar), and called Adar I, while the month simply called Adar in other years is called Adar II. Thus Purim (the 14th day of the twelfth month) is celebrated in the final month of any year rather than the twelfth as such.
During the diaspora (dispersion), this calendar was of great help, but it is no longer necessary because of both the possibility of spotting the new moon in Israel and instant communication such as via telephone or the internet.
While the contrived calendar has been amazingly accurate for having been set so many centuries ago, it still occasionally does not line up with the Aviv and often sets the new moon a day or two off from the actual sighting. Therefore, some, like ourselves, have returned to the ancient method of observing the holidays based on the actual new moon sighting and the Aviv. A thirteenth month is simply added whenever the Aviv stage of barley has not been reached by the end of the twelfth month. Since we do not know until a few days beforehand whether there will be a thirteenth month, should Purim be celebrated in the actual twelfth month every year? Not necessarily; it is not a commanded festival, and the book of Esther says it is for the Jews and any who wish to celebrate with them, so we feel that the date in this case should be determined by the Jewish calendar. The same goes for Hanukkah, which is a specifically-Jewish festival.
But otherwise, we must sometimes distinguish between the Biblical calendar and the Jewish calendar as used today, though the new Sanhedrin in Jerusalem has on its agenda a discussion of the return to the sighting of the new moon as the defining factor for the calendar. Of course this makes dates much less predictable, but the Hebrew way is to be ready to pull up stakes every time the Ark of the Covenant and the cloud of Yahweh's presence move.
The two types of holy days prescribed in Scripture are the hag (which means "to move or dance in a circle") and the moed ("appointed time" or "appointment", based on the word for "to bear witness" since it bears witness to us of who YHWH is and makes us part of the witness as we participate). Hagim are a subset of the moadim.
Hag gives us a clue that we should not think of time as a straight line, but as an upward spiral that keeps passing the same points as it heads back toward the Garden of Eden--a level higher and a step closer each time if we stay on track!