There is a principle regarding the Jewish calendar system that says, "Lo Adu Rosh," meaning, the first day of Rosh Hashanah cannot fall on Sunday, Wednesday or Friday. Likewise, there is a principle known as "Lo Badu Pesach," which means that the first day of Pesach cannot fall on Monday, Wednesday or Friday. The reason why the Rabbis ordained that Pesach cannot fall on these days is because if Pesach would fall on Monday, Wednesday or Friday, then the first day of Rosh Hashanah would fall on Sunday, Wednesday or Friday. And the Rabbis wanted to prevent these scenarios because of their ramifications regarding Yom Kippur: if the first day of Rosh Hashanah falls on Wednesday, then Yom Kippur falls on Friday, and if Rosh Hashanah falls on Friday, then Yom Kippur falls on Sunday. Either situation – where Yom Kippur falls on Friday or Sunday – would pose considerable difficulty, and the Rabbis therefore arranged the calendar in such a way that this could never happen.
It should be noted that these rules took effect only when the Rabbis established the fixed calendar system that we use today. In ancient times, however, when new months were declared based upon testimony to the sighting of the new moon, the Rabbis allowed Pesach to fall on Monday, Wednesday or Friday. Under this system, it was possible for the Rabbis to adjust the Rosh Chodesh declaration in the months prior to Rosh Hashanah as necessary to ensure that it would not fall on Sunday, Wednesday or Friday; they therefore had no need to prevent Pesach from beginning on a Monday, Wednesday or Friday. Only once the fixed calendar system came into use was it necessary to ensure that Pesach would not begin on these days.
This point is made by Rashi in his commentary to Masechet Pesachim (58). The Gemara records a Berayta stating that when Erev Pesach fell on Shabbat, the daily Tamid offering was brought in the Temple at the same time in the afternoon as it was "on Monday," meaning, on a normal day. Rashi raised the question of why the Berayta selected specifically Monday as an example of a normal day, rather than Sunday, and he brings an explanation that Erev Pesach cannot ever occur on Sunday, because the first day of Pesach cannot fall on Monday. But Rashi rejects this explanation, noting that the Berayta obviously refers to the period when the Temple stood, during which the first day of Pesach could, indeed, fall on Monday. Rashi therefore concludes that the Berayta chose Monday as an arbitrary example, without any particular reason.
