Yom Tov Sheni - Seeking All Relevant Sources

Question

There is an article on Wikipedia about Passover that gives a very non-traditional (if not anti-traditional) explanation - rather, a series of explanations - for the origin of Y"T Sheni Shel Galuyoth. I find the explanation(s) very weak (and actually offensive). The very next point in the article is about Karaite/Samaritan practice, and it takes a very authoritative stance on the dating of Passover from a Karaite/Samaritan point of view. It seems highly likely that this whole section (or a good portion of it) was written by a Karaite/Samaritan. Can someone please provide all the relevant sources for Y"T Sheni from the Halachic perspective, so that I (or someone else with a similar desire) can update the page? If someone else is a more senior Wikipedia expert (I've written some stubs and made a few edits, but never anything major like this) and would like to take it upon themselves to fix this, please do so. Please also share with the Mi.Yodea community if/when/how you intend to take on this task.

UPDATE - REQUEST: Can someone else with more Wikipedia experience and savvy take the lead on this? Since posting I have had very little time to devote to this, and I would probably need to devote a lot of time to ensure I do it in accordance with Wikipedia's standards.

Answer 1

The most complete sourcebook on Yomtov Sheni that I have seen is Rabbi Yerachmiel Fried's "Yomtov Sheini KiHilchasa." Here is a video shiur given by him on the topic .

Answer 2

The earliest source I know of that mentions two days of Yom Tov is the Mishnah (Eruvin 3:8), completed in the late second century. (Though to judge by the mishnayos before and after that, it may be talking about Rosh Hashanah specifically, where the reason we keep two days is somewhat different than for other Yamim Tovim.)

Some relevant passages from the Gemara:

"R' Yochanan [third century] announced: 'Anywhere that the messengers [sent by the Sanhedrin to inform people what day had been declared Rosh Chodesh] of Nissan can reach [before the holiday], but those of Tishrei cannot [- during Tishrei they have fewer days when they can travel, because of Rosh Hashanah and Yom Kippur] - must keep two days [of Pesach], as a precautionary measure against confusing the two [i.e., thinking that if Pesach need be kept for only one day, so does Sukkos].'" (Rosh Hashanah 21a)

"R' Yose [early fourth century] sent the following message to [the communities outside the Land of Israel]: 'Even though we have transmitted to you the rules for determining when the holidays occur [so that you need not wait for the monthly messengers to come, and you know on your own which days have been declared Rosh Chodesh], do not vary from the custom of your ancestors, may they rest in peace [who kept two days of each Yom Tov due to uncertainty].'" (Yerushalmi, Eruvin 3:9)

"What brought about that I have to keep two days of Yom Tov in Syria [or anywhere else outside of Israel]? The fact that I failed to keep one day in the Land [of Israel]." (ibid.)