Unlike the Roman calendar that begins its year in winter (in the northern hemisphere), the Bible calendar begins in spring (in the Middle East). In accordance with its ancient beginnings (right from Creation when light was created after the darkness), Bible calendar days were, and are, determined to begin and end at sunset.
Months of the Biblical calendar are linked directly to the phases of the moon (the word month actually means moon). The Bible month begins with the first sighting of the slender crescent of the new moon which does not become visible until a day or two after the precise time of the astronomical new moon, when the moon is directly between the earth and the sun. The moon at that time is usually slightly above or below the ecliptic. At other less common times, when the moon is directly in our line of sight to the sun, we get a solar eclipse (i.e. eclipses of the sun can occur only at the time of the new moon when the moon is between the earth and the sun. Conversely, eclipses of the moon can only occur at the time of the full moon when the earth is between the moon and the sun).
The Bible calendar is a very simple and natural means of keeping time. Although today commonly known as the "Jewish calendar," it actually long predates the Jewish people. God Himself gave it to all humanity. It uses His earth and His heavens as a great clock that can always be counted on - the earth keeps on rolling, and the moon always goes around.
Fact Finder: Were the sun and the moon to be used to mark days, months and seasons (years)?