Hi rav kornfeld,
Here's the question re:horayos. If you follow an erroneous ruling of the sanhedrin you're patur (the court and maybe the tribes bring the par haelem davar). But if you know them to be wrong and follow them you bring a chatas. How does this square with the story of rav Gamliel where rav yehoshua came and ate on the day he held to be Yom Kippur? Ie he followed a ruling he believed to be in error.
Thank you!
Joshua Danziger
Kidush ha'Chodesh is a special Din. The Mishnah in Rosh Hashanah (25a) derives from a verse that whether or not the day which Beis Din declares as Rosh Chodesh is correct or not, it automatically becomes Rosh Chodesh on the day that they announce. The Gemara there (25a) tells us that if the Beis Din erroneously, or even deliberately, proclaims that it is Rosh Chodesh on the wrong day, it nevertheless is effective.
The commentary of the Rambam (Kidush ha'Chodesh 2:10) writes that this is a special leniency for Kidush ha'Chodesh. We rely only on what the Beis Din say and this is the only way the month becomes Mekudash.
Therefore, even though Rav Yehoshua maintained that it was Yom Kippur, the Halachah was clear that it was not Yom Kippur.
Kol Tuv,
Dovid Bloom