Many assemblies take the crescent closest to the spring equinox, even if that crescent happens to fall before the equinox and call that "Abib one." Thus their holy days are one month too early. The correct method is to take the first crescent on or after the spring equinox and call that day "Abib one." (Other assemblies use "green ears" instead of the equinox. This method is synonymous with using either side of the equinox.)
Here are some of the reasons why it is correct to use the first crescent on or after the spring equinox:
1.) It is not consistent to have Abib one in the spring some years and in the winter other years. The festivals are to occur in their seasons, not out of them (Deut. 16:6; Num.9:2; Ex.13:10). Deuteronomy 16:1, Exodus 23:15 and 34:18 all make it clear that the moon must be "OF" the "GREEN EARS," not before them. There had to be enough barley developed for the wave sheaf (Lev.23:11).
2.) If Abib one can fall before the spring equinox, then Passover will always fall in spring but Trumpets, Atonement, Tabernacles and Last Great Day will fall mostly in summer, not autumn or fall. The fall holy days will be celebrated in two different seasons. Instead of harvesting crops during the harvest moon, people will have to travel to the feast empty-handed as their crops rot in the fields. If Abib one can fall 13 days before the equinox. then Tishri one (seventh month) will be 21 or 22 days before the fall equinox. It is not proper to have the harvest festivals in the summer (Lev.23:39).
3.) Farmers and shepherds two and three thousand years ago didn't know ahead of time if the equinox would be March 20, 21 or even 19. They didn't have almanacs. So if a crescent came 13, 14, or 15 days before, they wouldn't know if it were nearer or not ahead of time. Not only this, but also they couldn't predict ahead of time whether the month would have 29 or 30 days, further blurring the midpoint. And even if the equinox was March 20, and the crescent was 14 days before, the actual time of the equinox would probably fall many hours after the taking of the Passover on the night of the 19th. So even on the same day, Passover here falls short of spring. The ancients probably couldn't even calculate the equinox to the nearest day, let alone hour or minute. The equinox might fall at noon the 20th.
4.) The requirement of ripe barley for the wave sheaf (Lev.23:11), limits Nisan 15 to 21 to a time period from the beginning of April to early June (Solinsky, pp.46-48) "Barley begins to ripen in Palestine with the beginning of April, and in the lower and warmer parts the cutting is begun at the end of the same month. Hence we see that the first new moon, which began the first month and the Jewish year, could only take place in the last days of March at the earliest, and the sacrifice of the omer (wave sheaf) at the earliest only some days before the end of the first half of April". (Astronomy In The Old Testament, Giovanni V. Shiaparelli, 1905, Oxford). But if we use crescents 14 days before the equinox, Passover can fall as early as March 19th.
5.) The Messiah's last Passover was observed in a year when the crescent before the equinox was closer than the one after and yet Messiah used the one after. Only 28, 31, and 34 C.E. had Wednesday Passover dates, and only 31 C.E. is correct among these three. That being the case, the only Wednesday Passover of 31 C.E. was on April 25th. Fourteen days earlier makes April 11th which is 21 days after March 21st (latest possible equinox) showing that the nearer crescent before the equinox wasn't used.
6.) From the book Calendarium Palestini by William Carpenter, page 32, we read about the Passover and Days of Unleavened Bread: "It was celebrated on the 14th day of the moon next after the vernal equinox and continued seven days." (Written in 1825) Furthermore, we know that Ezra and Nehemiah used the Babylonian names for the months of the calendar. By all indications, they actually used the Babylonian calendar itself and knew no other. We know that the Babylonian calendar used visible crescents and also it did not allow the first month of the year to come before the vernal equinox for hundreds of years. No less an authority than Otto Neugebauer, knew of no archaeological evidence to the contrary that the Babylonian and Biblical calendars are the same. Also, according to the Babylonian Talmud, Sanhedrin, pp. 46 to 51, we find that Rabbon Simeon Ben Gamaliel (Paul's teacher) intercalated a month on three grounds: tekufah*, barley and roads. He said the most important method was tekufah. When they asked if the other two matter, Gamaliel made no reply because he would have been persecuted by the Pharisees if he had contradicted them.
* Tekufot (seasons). The four seasons in the Jewish year are called tekufot.
More accurately, it is the beginning of each of the four seasons, that is named tekufah ( literally "circuit", "to go round"), the tekufah of Abib denoting the mean sun at the point of the vernal equinox.