Why are our feast days, which are commanded in the Tanakh as Mo’edim or “Appointed Times,” oftentimes different from mainstream Judaism and many other Messianic congregations?
Answer: The short answer is, “We are not Rabbinic”. We do not follow the teachings of the Rabbis simply because they are Jewish, as the Rabbis were shown by Messiah to have erred from the Commandments. Our “Judaism” is 1st century, Messianic, “Mishnaic” Judaism, meaning the customs we follow were practiced by Messiah Yeshua and the Shlikhim [Apostles], and recorded in the 2nd century in what is known as “Mishnah,” confirming and explaining the ‘how’ of what is written in Torah. The Mishnah tells us they “observed”/”saw” the “khodesh”, the crescent new moon. This aligns with Torah. The current, Jewish schedule of holy days follows a calculated solar/lunar calendar which was not completely codified until nearly 1000AD, and uses Babylonian methods of setting feast days, having been first calculated by Hillel the 2nd in the sixth century AD. The Torah has no real concept of a fixed “month”, but counts each “khodesh”, or “renewed crescent moon”.
The start of the Mo’edim, or feasts, is set in the spring at Pesakh, each new crescent moon determining the benchmark for setting the following feasts. Pesakh is 14 days after the first crescent, and Yom Teruah/Rosh HaShannah falls on the 7th renewed crescent moon. For details on determining Pesakh and the subsequent feasts, please read this document: Reckoning Pesakh