Leviticus 23:4

These are the times of the Festivals of the Lord, holy convocations which you shall proclaim in their times:

Scripture now discusses festivals. The Sabbaths were called “ My chosen times” [:2]; the festivals are called their chosen time , because God does not determine the day of the week on which they fall. Allow me at this point to present a few general principles concerning the festivals. In the days when the Temple still stood, the Sanhedrin determined when the festivals would be held. Consider the case of King Hezekiah’s Passover, where Scripture clearly says “The king had taken counsel…” [II Chronicles 30:2]. The Exegetes tell us that the Sanhedrin would examine many factors when determining the length of the year. For example, Rabbi ⊂ Aqiba is said to have decreed two [actually three — Translator] leap years in a row [Sanhedrin 12a], because the situation required it. In all of Scripture we find no evidence of the procedure whereby ancient Israel maintained its calendar. Sa‘adya Gaon claims that they made the calendar conform to the same fixed rules that we now employ; but this cannot be true, as the Mishna and the Talmud both record that Passover occasionally used to fall on Monday, Wednesday, or Friday. Two stories tell of the occurrence of such events. Today we add a leap month whenever the lunar year falls approximately one month behind the solar year, but in ancient times this was not the case. The Jewish calendar began on a Friday morning at 8 o’clock (by adding 4 days, 8 hours, 48 minutes and 40 seconds to Sunday night, 11 minutes and 20 seconds past 11 o’clock in the evening, we get 8 o’clock Friday morning ), and the first intercalation occurred one and a half years later. Nowadays, the calendar is based on the average duration of the moon’s orbit. Although the interval between two appearances of the new moon is “sometimes longer and sometimes shorter” [Rosh HaShana 25a] than a lunar month, the actual sighting of the moon over Jerusalem (or over any other place) does not occasion the declaration of the new month. To prove this, assume that the midpoints of the sun, moon, and earth are all connected by a straight line. Is Jerusalem necessarily the place where this line intersects the earth’s surface? Moreover, whenever we see the new moon of Nisan on a Sunday night — and everyone around the world sees the new moon at the same time — the month of Nisan does not begin until Monday night. The months of Ṭevet, Sh e vaṭ, and Adar can likewise be delayed one day after the sighting of the new moon. This can happen whenever the New …