Study Rosh Hashanah folio 20A with parallel Hebrew-English text, traditional commentary, and modern study tools. Free access to Babylonian Talmud online.
אלא אי אמרת לעולם חסר, אמאי מחללינן? משום דמצוה לקדש על הראייה.
But if you say that the Adar immediately preceding Nisan is always short, why should they desecrate Shabbat? The court can calculate the time of Rosh Chodesh without witnesses. The Talmud answers: Because it is a mitzva to sanctify Rosh Chodesh on the basis of the testimony of witnesses who actually saw Rosh Chodesh and not rely on calculations or established practices.
איכא דאמרי, אמר רב נחמן, אף אנן נמי תנינא: על שני חדשים מחללין את השבת — על ניסן ועל תשרי. אי אמרת בשלמא לעולם חסר — משום הכי מחללינן, דמצוה לקדש על הראייה.
Some say that Rav Naḥman said: We, too, learn in a Mishnah: Witnesses who saw Rosh Chodesh may desecrate Shabbat to establish Rosh Chodesh for two months, for the month of Nisan and for the month of Tishrei. Granted, if you say that the Adar immediately preceding Nisan is always short, due to that reason the witnesses may desecrate Shabbat, because it is a mitzva to sanctify the month on the basis of the testimony of witnesses who actually saw Rosh Chodesh.
אלא אי אמרת זמנין מלא זמנין חסר — אמאי מחללינן? נעבריה האידנא, ונקדשיה למחר.
But if you say that the Adar immediately preceding Nisan is sometimes full and sometimes short, and there is no regularity to it, why should the witnesses desecrate Shabbat? Have the court add an extra day to the month now, since it can decide whether the 30th day or the 31st day is the beginning of the new month, and let the members of the court sanctify the next day as Rosh Chodesh.
אי דאקלע יום שלשים בשבת — הכי נמי. הכא במאי עסקינן: דאקלע יום שלשים ואחד בשבת, דמצוה לקדש על הראייה.
The Talmud rejects this argument: If the 30th of Adar falls on Shabbat, this is indeed what is done. The witnesses are not permitted to desecrate Shabbat. Rather, the month is made full and the next day is sanctified as Rosh Chodesh. Here we are dealing with a case where the 31st of Adar falls on Shabbat, and so the sanctification of Rosh Chodesh cannot be pushed off to the next day, as a month cannot be longer than 30 days. In that case, the witnesses may desecrate Shabbat because it is a mitzva to sanctify the month based on testimony of witnesses who actually saw Rosh Chodesh.
Text data via Sefaria: William Davidson Edition - English and William Davidson Edition - Vocalized Aramaic, CC-BY-NC; edition provenance: Koren. Display terminology has been modernized for readability.