daf bit: Beitzah 4
This leads to a discussion of why we keep a second festival day now when there is no longer any uncertainty about when a month begins. Abaye said: in early times they used to light bonfires to signal that the new moon had been sighted, but because of the mischief of the Samaritans (who sought to disrupt Jewish observance) the rabbis ordained that messengers should go forth, which takes time, so the extra day is insurance because the messenger might not have gotten to you yet. But, Abaye continued, if the mischief of the Samaritans were to cease and we went back to bonfires we would only keep one day. Further, even when the Samaritans were causing trouble, if the messenger arrived in time we kept only one day. So why do we observe two days now when we are well-acquainted with the calendar and don't need to wait for anybody? The answer: because they sent word from Palestine: give heed to the customs of your ancestors which have come down to you, for it might happen that the government will issue a decree and it will cause confusion in ritual. (4b)
So, we keep two days of festivals even though we don't need to on account of the calendar itself, because of tradition and because some successor to the Samaritans could mess with us again. I knew the "tradition" argument but not the other, and I didn't know this was explicitly addressed in talmud. I thought it was later rabbis who said that we have to keep doing it because we don't have a Sanhedrin that can override this law.
That the arrival of the messengers trumped the extra day (once the custom of the extra day was established) is something I did not know.

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And yes, of course tangents. :-)