Pessah on a Sunday is a rare occurrence and includes unique laws that depart from the usual pre-Pessah routine since Erev Pessah falls on a Shabbat. The last time we had such a constellation was in 2005 and we can expect it again only in 13 years from now, in 2021. Similarly between 1981 and 1994, 13 years passed without a Sunday Pessah, but according to our current calendar this break can at times extend to as much as 20 years. For example, between 1954 and 1974, Pessah never fell on a Sunday, and it will not happen again between 2025 and 2045, assuming we continue to operate under the same fixed calendar. In days of old, when the new month was set by witnesses who testified to seeing the new moon - not by a fixed calendar - there was no telling how many years could pass without a Sunday Pessah. Rabbinic literature recounts an episode that occurred in such a year when Erev Pessah fell on Shabbat (T. Pessahim 4:13-14; B. Pessahim 66a; Y. Pessahim 33a). The sons of Beteira served in the office of the Nassi during the Second Temple period. When Erev Pessah once fell on Shabbat, they could not remember whether the Pessah sacrifice, which is offered on the 14th of Nisan and eaten on the 15th at the Seder, was to override the Shabbat prohibitions against slaughtering animals and offering them as sacrifices. Perhaps with a tone of despair, they asked: "Is there no one who knows whether the Pessah sacrifice takes precedence over Shabbat or not?" "There is one person, who has come to the Land of Israel from Babylon, and Hillel the Babylonian is his name. He attended the two greats of the previous generation, Shemaya and Avtalyon. And he knows whether or not the Pessah sacrifice overrides the Shabbat prohibitions." The sons of Beteira promptly sent for Hillel, and with some urgency asked him: "Does the Pessah sacrifice override Shabbat?" Without directly answering the question, and perhaps with a smug grin on his face, Hillel responded cryptically: "Is there only one Pessah sacrifice a year that overrides Shabbat? Are there not more than 200 such sacrifices a year that override Shabbat?" Hillel appears to be referring to the sacrifices that are offered on each Shabbat. On a regular Shabbat four animals are sacrificed, reaching a total of 200 sacrifices over 50 weeks of the Jewish year. Add to that number additional offerings brought on a Shabbat that occurs during the intermediate days of Pessah and of Succot - seven and 14 animals respectively - the total is, as Hillel said, more than 200. Thus sacrifices are offered as part of the Shabbat Temple ritual, and there is no reason that the Pessah sacrifice should be any different. In plain words: The Pessah sacrifice overrides Shabbat prohibitions. The sons of Beteira, perhaps irked by Hillel's tone, were unsatisfied: "What is your source?" Adding to his logical argument, Hillel offered two verifications based on hermeneutical principles of exegesis of the biblical text. The first proof cites the method known as gezeira shava - when the same word is used in two different contexts, certain laws associated with one context may be applied to the other. In our case, the word mo'ado appears in the context of the Pessah sacrifice (Numbers 9:2) and in the context of the daily Tamid sacrifice (Numbers 28:2): Just as the daily sacrifice is offered on Shabbat, so too the Pessah sacrifice should be offered on Shabbat. The second corroboration cited by Hillel cites a kal vahomer, an a fortiori argument: Since the daily sacrifice overrides Shabbat even though the punishment for not bringing this sacrifice is not the serious karet, surely the Pessah sacrifice that carries this severe penalty should override Shabbat! According to one version of the tale, Hillel was appointed nassi without delay and took the place of the sons of Beteira. In this new role, he immediately began to publicly teach the laws of Pessah. An alternative version is less charmed with Hillel's prowess. His answers are essentially rejected with counter-hermeneutical arguments. Even though Hillel continues to press his opinions, the Beit Midrash remains unconvinced. Finally, Hillel declares that this was the ruling he received from his teachers, the sages Shemaya and Avtalyon. At this point, the Beit Midrash concedes his greatness and grants him the title of nassi. Once Hillel takes office, he begins to scold those he was teaching: "What caused you to need a Babylonian to come up to the Land of Israel to serve as nassi over you? Your laziness! In that you did not sufficiently attend the two giants, Shemaya and Avtalyon." In the wake of this reproach, Hillel was asked the following question: If one forgot to bring the slaughtering knife before Shabbat, what should he do so that he may still slaughter his Pessah sacrifice on Shabbat as required? With all eyes turned to him, Hillel responded, perhaps nervously stammering: "That law... I heard... but... I forgot." One version of the tale merely hints at a correlation between Hillel's haughty rebuke and his sudden onset of forgetfulness. Another version unsympathetically makes this connection outright. Quickly covering his tracks, Hillel says: "Rely on Israel, for if they are not prophets, they are surely the children of prophets." Thus Hillel expressed his trust that the masses would act appropriately. The next day, on Shabbat the 14th of Nisan, whoever had a sheep for a Pessah sacrifice shoved the knife into its wool, and whoever had a goat wedged the knife between its horns. As soon as Hillel saw this, he recalled the law and declared: "Indeed, thus I received from Shemaya and Avtalyon." The sporadic incidence of Erev Pessah falling on Shabbat and the different practices this entails means that many of us may not remember the distinct issues that are raised by a Sunday Pessah: When do the firstborns fast? When should we dispose of the hametz? What should we eat on Shabbat? Though we may forget a few of the intricate laws, we may take comfort in knowing that we are in esteemed company. The writer is on the faculty of Pardes Institute of Jewish Studies and is a rabbi in Tzur Hadassah.