Never mind the summer solstice: for Jews around the world, the longest day of the year begins at sundown and lasts a full 48 hours. Although outlined in the Bible and early Jewish sources as a single day of ram’s horn blowing and temple consecrating, to be held on the first day of the autumn month of Tishrei, Rosh Hashanah eventually grew into a two-day observance (technically a “Yoma Arichta,” from the Aramaic for “long day”).

When first mandated in the Torah (Leviticus 23:24), what became Rosh Hashanah was an unnamed minor festival held on the first day of the seventh month, serving mainly as a precursor to the major harvest celebration of Sukkot two weeks later. Although Rosh Hashanah means “beginning of the year,” in that era the new year was generally calculated from the beginning of the month of Nisan, in the spring.

The earliest known descriptions of Rosh Hashanah as we now know it come from the oral traditions compiled and organized in the Mishnah, around 200 C.E. By then the holiday was regularly celebrated over two days because of the uncertainty of predicting which day the sighting of the new moon—which signaled the start of the new month—would be announced by the Sanhedrin, the central Jewish religious court. Because it took time for news of the Sanhedrin’s announcements to travel and observant Jews needed to avoid even accidental work on the festival day, the two-day Rosh Hashanah observance began as a safeguard. Today Rosh Hashanah is the only holiday celebrated for two days both inside and outside of Israel.