(The following is a cross-post by Sharon Horowitz, reference librarian in the Hebraic Section of the African and Middle Eastern Division. It originally appeared on the Library of Congress blog.)
Exodus 23:15 tells us that Passover should be celebrated in the spring. The rabbis understood this to mean it was their job to maintain the holiday in the spring, which required some manipulation of the lunar-based Jewish calendar because the lunar calendar has fewer days than the solar year. Were it not for the intervention of t