Most Christians know that Jesus was crucified in Jerusalem at the time of the Passover, and in this way became the perfect Passover sacrifice, "the Lamb of God which taketh away the sin of the world" (John 1:29). Since the early centuries of the church, Christians have honored the death and resurrection of Jesus in the celebration of Easter. And while the observance of Easter has changed over the centuries, it is based on the Hebrew Passover.

Hebrew Roots

We read in the Book of Exodus how the LORD used Moses lead the Israelites out of Egypt. In Exodus 12, the LORD sent the final plague, smiting the firstborn of all Egypt. The Israelites were saved by sacrificing a lamb and covering their doorframes with its blood, so that the LORD would "pass over" the house and not smite the firstborn of Israel. And the LORD commanded that the Israelites remember the Passover in a seven-day feast....

"And this day shall be unto you a memorial; and ye shall keep it a feast to the LORD throughout your generations; ye shall keep it a feast by an ordinance for ever." - Exodus 12:14. 

The LORD instructed Israel as to when the Passover should be kept: "In the first month, on the 14th day of the month at even, ye shall eat unleavened bread, until the one and 20th day of the month at even." - Exodus 12:18.

In the lunar calendar, the New Moon is the first day of the new month. The Full Moon is at mid-month, the 14th day of the month. So Israel was instructed to celebrate the Passover on the Full Moon of the first month. The LORD even tells us the name of the month....

"Observe the month of Abib, and keep the passover unto the LORD thy God: for in the month of Abib the LORD thy God brought thee forth out of Egypt by night." -Deuteronomy 16:1

The word "Abib" means "sprouting" or "budding" and is the first month of Spring, falling among our months of March and April. Abib is the only month of the year named by God in the books of Moses.  

The Bible doesn't clearly indicate the calendar method used to keep track of Passover and other Hebrew holidays. However, for centuries the Jewish calendar has been based on the well-known, 19-year cycle of the Sun and Moon. God in His providence established that 235 cycles of the Moon's phases is nearly equal to 19 solar years. This means that every 19 years, the Moon's phases will recur on the same dates of the solar year.

This 19-year "luni-solar" cycle was the basis for the calendar used in Babylon. In the west, its discovery is attributed to the Greek philosopher Meton, who may have learned it from the Babylonians. The 19-year cycle was generally well known and understood in antiquity, and was apparently used by the Persian rulers of Babylon after the restoration of Israel. 

It appears that the modern Jewish calendar was directly influenced by the Babylonian calendar. In addition to using the 19-year cycle, the modern Jewish calendar uses months with names nearly identical to the Babylonian names. In the books of Nehemia, Ezra and Esther (which record events after the Babylonian exile) you can find the month names "Sivan," "Elui," and "Adar," nearly the same as their Babylonian counterparts. 

Most notable is the month "Nisan," the first month of Spring, corresponding to the Mosaic month of "Abib." In the modern Jewish calendar, Passover occurs on the fourteenth day of Nisan. The Jewish historian Flavius Josephus, writing in about 90 A.D., gives us an astronomical reference for the month of Nisan....

"In the month of Xanthicus, which is by us called "Nisan," and is the beginning of our year, on the 14th day of the lunar month, when the Sun is in Aries (for in this month it was that we were delivered from bondage under the Egyptians, the law ordained that we should every year slay the sacrific which I before told you we slew when we came out of Egypt, and which was called the Passover....)"