Shane, you bring up an interesting point. Present knowledge of the Jewish calendar in use before the period of the Babylonian Exile is both limited and uncertain. The Bible refers to calendar matters only incidentally, and the dating of components of Mosaic Law remains doubtful. The earliest datable source for the Hebrew calendar is the Gezer Calendar, written probably in the age of Solomon, in the late 10th century BC. The inscription indicates the length of main agricultural tasks within the cycle of 12 lunations. The calendar term here is yereah, which in Hebrew denotes both "moon" and "month." The second Hebrew term for month, hodesh, properly means the "newness" of the lunar crescent. Thus, the Hebrew months were lunar. They are not named in pre-exilic sources except in the biblical report of the building of Solomon's Temple in I Kings, where the names of three months, two of them also attested in the Phoenician calendar, are given; the months are usually numbered rather than named. The "beginning of the months" was the month of the Passover. In some passages, the Passover month is that of hodesh ha-aviv, the lunation that coincides with the barley being in the ear. Thus, the Hebrew calendar is tied in with the course of the Sun, which determines ripening of the grain. It is not known how the lunar year of 354 days was adjusted to the solar year of 365 days. The Bible never mentions intercalation. The year shana, properly "change" (of seasons), was the agricultural and, thus, the religious year. There is no reference to the New Year's day in the Bible.
After the conquest of Jerusalem (587 BC), the Babylonians introduced their cyclic calendar and the reckoning of their regnal (dating from the ascention of a king) years from Nisanu 1, about the spring equinox. The Jews now had a finite calendar year with a New Year's day, and they adopted the Babylonian month names, which they continue to use. From 587 BC until AD 70, the Jewish civil year was Babylonian, except for the period of Alexander the Great and the Ptolemies (332-200 BC), when the Macedonian calendar was used. The situation after the destruction of the Temple in Jerusalem in AD 70 remains unclear. It is not known whether the Romans introduced their Julian calendar or the calendar that the Jews of Palestine used after AD 70 for their business transactions. There is no calendar reference in the New Testament; the contemporary Aramaic documents from Judaea are rare and prove only that the Jews dated events according to the years of the Roman emperors. The abundant data in the Talmudic sources concern only the religious calendar.
In the religious calendar, the commencement of the month was determined by the observation of the crescent New Moon, and the date of the Passover was tied in with the ripening of barley. The actual witnessing of the New Moon and observing of the stand of crops in Judaea were required for the functioning of the religious calendar. The Jews of the Diaspora, or Dispersion, who generally used the civil calendar of their respective countries, were informed by messengers from Palestine about the coming festivals. This practice is already attested for 143 BC. After the destruction of the Temple in AD 70, rabbinic leaders took over from the priests the fixing of the religious calendar. Visual observation of the New Moon was supplemented and toward AD 200, in fact, supplanted by secret astronomical calculation. But the people of the Diaspora were often reluctant to wait for the arbitrary decision of the calendar makers in the Holy Land. Thus, in Syrian Antioch in AD 328-342, the Passover was always celebrated in (Julian) March, the month of the spring equinox, without regard to the Palestinian rules and rulings. To preserve the unity of Israel, the patriarch Hillel II, in 358/359, published the "secret" of calendar making, which essentially consisted of the use of the Babylonian 19-year cycle with some modifications required by the Jewish ritual.
Which brings us to the relationship between the Jewish calendar and our present calendar.
In the mid-1st century BC Julius Caesar invited Sosigenes, an Alexandrian astronomer, to advise him about the reform of the calendar, and Sosigenes decided that the only practical step was to abandon the lunar calendar altogether. Months must be arranged on a seasonal basis, and a tropical (solar) year used, as in the Egyptian calendar, but with its length taken as 365 1/4 days.
To remove the immense discrepancy between calendar date and equinox, it was decided that the year known in modern times as 46 BC should have two intercalations. The first was the customary intercalation of the Roman republican calendar due that year, the insertion of 23 days following February 23. The second intercalation, to bring the calendar in step with the equinoxes, was achieved by inserting two additional months between the end of November and the beginning of December. This insertion amounted to an addition of 67 days, making a year of no less than 445 days and causing the beginning of March, 45 BC in the Roman republican calendar, to fall on what is still called January 1 of the Julian calendar.
Apparently, the Pontifices misinterpreted the edict and inserted an extra day for "leap year" too frequently. The error arose because of the Roman practice of inclusive numbering, so that an intercalation once every fourth year meant to them intercalating every three years, because a bissextile (leap) year was counted as the first year of the subsequent four-year period. This error continued undetected for 36 years, during which period 12 days instead of nine were added. The emperor Augustus then made a correction by omitting intercalary days between 8 BC and AD 8. As a consequence, it was not until several decades after its inception that the Julian calendar came into proper operation, a fact that is important in chronology but is all too frequently forgotten.
It seems that the months of the Julian calendar were taken over from the Roman republican calendar but were slightly modified to provide a more even pattern of numbering. The republican calendar months of March, May, and Quintilis (July), which had each possessed 31 days, were retained unaltered. Although there is some doubt about the specific details, changes may have occurred in the following way. Except for October, all the months that had previously had only 29 days had either one or two days added. January, September, and November received two days, bringing their totals to 31, while April, June, Sextilis (August), and December received one day each, bringing their totals to 30. October was reduced by one day to a total of 30 days and February increased to 29 days, or 30 in a bissextile year. With the exception of February, the scheme resulted in months having 30 or 31 days alternately throughout the year. And in order to help farmers, Caesar issued an almanac showing on which dates of his new calendar various seasonal astronomical phenomena would occur.
These arrangements for the months can only have remained in force for a short time, because in 8 BC changes were made by Augustus. In 44 BC, the second year of the Julian calendar, the Senate proposed that the name of the month Quintilis be changed to Julius (July), in honour of Julius Caesar, and in 8 BC the name of Sextilis was similarly changed to Augustus (August). Perhaps because Augustus felt that his month must have at least as many days as Julius Caesar's, February was reduced to 28 days and August increased to 31. But because this made three 31-day months (July, August, and September) appear in succession, Augustus is supposed to have reduced September to 30 days, added a day to October to make it 31 days, reduced November by one day to 30 days, and increased December from 30 to 31 days, giving the months the lengths they have today.
Which brings us to the Gregorian calendar.
The need for additional modifications became apparent some time soon after the 6th century as a result of developments in calendar computation in the previous 200 years. In AD 463, Victorius (or Victorinus) of Aquitaine, who had been appointed by Pope Hilarius (love that name!

) to undertake calendar revision, devised the Great Paschal (i.e., Passover) period, sometimes later referred to as the Victorian period. It was a combination of the solar cycle of 28 years and the Metonic 19-year cycle, bringing the Full Moon back to the same day of the month, and amounted to 28 X 19, or 532 years. In the 6th century, this period was used by Dionysius Exiguus (Denis the Little) in computing the date of Easter, because it gave the day of the week for any day in any year, and so it also became known as the Dionysian period. Dionysius took the year now called AD 532 as the first year of a new Great Paschal period and the year now designated 1 BC as the beginning of the previous cycle. In the 6th century it was the general belief that this was the year of Christ's birth, and because of this Dionysius introduced the concept of numbering years consecutively through the Christian Era. The method was adopted by some scholars but seems only to have become widely used after its popularization by the Venerable Bede of Jarrow (673-735), whose reputation for scholarship was very high in Western Christendom in the 8th century. This system of BC/AD numbering threw into relief the different practices, or styles, of reckoning the beginning of the year then in use. When the Gregorian calendar firmly established January 1 as the beginning of its year, it was widely referred to as the New Style calendar, with the Julian the Old Style calendar. In Britain, under the Julian calendar, the year had first begun on December 25 and then, from the 14th century onward, on March 25.
Our calendar was again reformed in the mid 18th century, largely a result of the French revolution, to standardize the first day of the new year as January 1 in stead of March 1st. This happened in the US in 1777. (This also explains why the name of the 9th month comes from the Latin word for "seven" - the 10th month comes from the Latin word for "eight" - the 11th month comes from the Latin word for "nine" and the 12th month comes from the Latin word for "ten!").
So, the bottom line is, we really don't know what day of the week it is today in the Jewish calendar. So we go with what we have and don't worry about it, remembering Colossians 2:16.
[much of the above information comes from my syllabus on ancient dating and was gathered from several sources including the OED and EB]