Intercalation or Bust: A Scriptural Calendar Throwdown

(1) יהוה said to Moses and Aaron in the land of Egypt: (2)This month shall mark for you the beginning of the months; it shall be the first of the months of the year for you.

(14) This day shall be to you one of remembrance: you shall celebrate it as a festival to יהוה throughout the ages; you shall celebrate it as an institution for all time. (15) Seven days you shall eat unleavened bread; on the very first day you shall remove leaven from your houses, for whoever eats leavened bread from the first day to the seventh day, that person shall be cut off from Israel.

(3) And Moses said to the people,“Remember this day, on which you went free from Egypt, the house of bondage, how יהוה freed you from it with a mighty hand: no leavened bread shall be eaten. (4) You go free on this day, in the month of Aviv.

(4) These are the set times of יהוה, the sacred occasions, which you shall celebrate each at its appointed time: (5) In the first month, on the fourteenth day of the month, at twilight, there shall be a passover offering to יהוה, (6) and on the fifteenth day of that month יהוה’s Feast of Unleavened Bread. You shall eat unleavened bread for seven days. (7) On the first day you shall celebrate a sacred occasion: you shall not work at your occupations. (8) Seven days you shall make offerings by fire to יהוה. The seventh day shall be a sacred occasion: you shall not work at your occupations.

(1) Observe the month of Aviv and offer a passover sacrifice to your God יהוה, for it was in the month of Aviv, at night, that your God יהוה freed you from Egypt.

(9) Upon hearing that Rabbi Yehoshua had challenged his ruling, Rabban Gamliel sent a message to him: I decree against you that you must appear before me with your staff and with your money on Yom Kippur when it occurs according to your calculation; according to R' Gamliel's calculation, that day was the eleventh of Tishrei, the day after Yom Kippur. Rabbi Akiva was in a pinch. because the head of the Great Sanhedrin was forcing R' Yehoshua to desecrate the day that he maintained was Yom Kippur. In an attempt to console him, Rabbi Akiva said tohim: I have learned from a verse that everything that Rabban Gamliel did is done, i.e., his opinion is valid. As it is stated: “These are the appointed seasons of the Lord, sacred convocations, which you shall proclaim in their season” (Leviticus 23:4). This verse indicates that whether you have proclaimed them at their proper time or whether you have declared them not at their proper time, I have only these Festivals as established by the representatives of the Jewish people. Rabbi Yehoshua then came to Rabbi Dosa ben Horkinas, who said to him: If we come to debate and question the rulings of the court of Rabban Gamliel, we must debate and question the rulings of every court that has stood from the days of Moses until now. ...As it is stated: “Then Moses went up, and Aaron, Nadav and Avihu, and seventy of the Elders of Israel” (Exodus 24:9). But why were the names of these seventy Elders not specified? Rather, this comes to teach that every set of three judges that stands as a court over the Jewish people has the same status as the court of Moses. Since it is not revealed who sat on that court, apparently it is enough that they were official judges in a Jewish court. When Rabbi Yehoshua heard that even Rabbi Dosa ben Horkinas maintained that they must submit to Rabban Gamliel’s decision, he took his staff and his money in his hand, and went to Yavne to Rabban Gamliel on the day on which Yom Kippur occurred according to his own calculation. Upon seeing him, Rabban Gamliel stood up and kissed him on his head. He said to him: Come in peace, my teacher and my student. You are my teacher in wisdom, as Rabbi Yehoshua was wiser than anyone else in his generation, and you are my student, as you accepted my statement, despite your disagreement.

(1) The months of the year are lunar months, as [implied by Numbers 28:14]: "... the burnt offering of the month when it is renewed," and [Exodus 12:2]: "This month shall be for you the first of months." [Concerning this verse,] our Sages commented: The Holy One, blessed be He, showed Moses in the vision of prophecy an image of the moon and told him, "When you see the moon like this, sanctify it." The years we follow are solar years, as [implied by Deuteronomy 16:1]: "Keep the month of spring."


(2) How much longer is a solar year than a lunar year? Approximately eleven days. Therefore, [to correct the discrepancy between the lunar and the solar calendars,] when these additional days reach a sum of 30 - or slightly more, or slightly less - an additional month is added, causing the year to include thirteen months. This is called a full year. [This is necessary,] because it is impossible to have a year with twelve months and an odd number of days, as [implied by Numbers 28:14]: "... of the months of the year." [On this verse, our Sages commented:] "You count the months of a year, but not the days of a year."


(3) The moon becomes hidden and cannot be seen for approximately two days - or [slightly] less or slightly more - every month: approximately one day before its conjunction with the sun at the end of the month, and one day after its conjunction with the sun, [before] it is sighted in the west in the evening. The first night when the moon is sighted in the west after being hidden is the beginning of the month. Afterwards, 29 days are counted from that day. If the moon is sighted on the night of the thirtieth [day], the thirtieth day will be Rosh Chodesh [of the following month]. If it is not sighted, Rosh Chodesh will be on the thirty-first day, and the thirtieth day will be included in the previous month. There is no need [to sight] the moon on the thirty-first night; whether or not [the moon] is sighted [the new month begins that night]. For there are no lunar months longer than thirty days.