I gave a Shiur over Shavuot on Yom Tov Sheini as it relates to Sefirat Ha’Omer and Shavuot. Here are the sources. A few notes based on the sources:
1) Sources 18 and 19 provide a fascinating back-and-forth between the Ba’al Ha’Maor and the Ra’avad (both commentaries on the Rif) regarding the custom outside Israel to count Sefirah with a Beracha on the second night of Pesach (16 Nissan) – a night when, as shown by our second Seder, we believe that it might still be 15 Nissan. The Ba’al HaMaor in Source 18 suggests we count the first night of Sefirah without a Beracha, while the Ra’avad in Source 19 retorts that, according to the Ba’al HaMaor’s own logic, his solution would not solve the underlying problem because the count itself (with or without a Beracha) indicates that it is now definitely 16 Nissan. The Ra’avad, however, is not bothered by the whole issue because to him, the Ba’al HaMaor’s premise is rooted in a fallacy that the Beracha or count indicate a definite date, while the Ra’avad himself believes that there is nothing inherent in either the Beracha or the count to indicate that it is a particular date. The Ba’al HaMaor’s own answer focuses on the need to save a דאורייתא in the face of an attack by a דרבנן: on Shemini Atzeret, we don’t make a Beracha in the Succah because the דאורייתא of Shemini Atzeret is under attack by the דרבנן of sitting in a Succah on what is most probably not Hoshanah Rabbah. Likewise, the דרבנן of a second Seder is not strong enough to attack the דאורייתא of doing Sefirat HaOmer with a Beracha on what is most likely (OK, definitely is) 16 Nissan, or the דאורייתא of celebrating the first night of Shavuot (which, if we counted מִסָפֵק, would be Night #49, thus trivializing the first night of Shavuot).
I wanted to offer another possible answer, one probably closer to the Ba’al HaMaor than to the Ra’avad (the latter of whose own logical fallacy is somewhat obtuse – Sefirah #1 is scheduled for ממחרת השבת, and everyone in the world knows that’s 16 Nissan … wait, is the Ra’avad a Tzaduki?!). If we consider all of the “doubty” things we do on Yom Tov Sheini – Kiddush, Hallel, and Succah come easily to mind – all of them have one thing in common: the Mitzvot themselves appear wholly intact, while a doubt exists in the mind of the performer of the Mitzvah. Counting Sefirah without a Beracha, or counting two numbers each night (another suggestion by the Ba’al HaMaor), would cross a red line because now the doubt would exist explicitly rather than implicitly; the doubt would be in the Mitzvah itself, not in the mind of the performer of the Mitzvah. Counting מִסָפֵק would create a doubt in the חפצא של מצוה, the object of the Mitzvah, rather than in the גברא, the one performing the Mitzvah. Perhaps we allow Yom Tov Sheini insofar as it expresses doubts related to our individual relationship with the Mitzvah, but not insofar as it expresses doubts inherent in the Mitzvot themselves.
2) I have always been partial to the belief that it is better to count Sefirah after the Seder, rather than in Shul at the end of Maariv, and that it is for this reason that many Haggadot print Sefirah right before Nirtzah. If, as rumored, the Haggadot printed Sefirah for the benefit of women and children who were not in Shul, why would every single Haggadah that prints Sefirah put it way in the back, right after Hallel, when so many people are already asleep, rather than printing it before Kadesh so that people can count as early in the evening as possible? I think Sefirah in the Haggadah is not printed for women and children but is there for everyone to say immediately after the final Mitzvah of the night, Hallel, is completed, at which point we can quietly state that, indeed, we are aware that it is 16 Nissan, not 15. Counting anytime earlier would create an internal contradiction (a תרתי דאסאתרי) in that we have already declared it to be 16 Nissan but are now performing Mitzvot unique to 15 Nissan. Better to complete the Seder and then count, rather than count and then perform the many Mitzvot of the Seder in a state of explicit disharmony.
The Ra’avad (Source 19), however, feels differently. At the end of his response to the Ba’al HaMaor (beginning with “ועוד”), he gives another reason why we should count with a Beracha on the second night of Pesach. Invoking the concept of מעלין בקודש ואין מורידין, the Ra’avad points out that there is no problem making a day Holy (i.e., by making a Seder and declaring that it is 15 Nissan) after it is has already been made non-Holy (i.e., by counting Sefirah and declaring that it is 16 Nissan). The problem, he says, would only apply in reverse – making a Seder and then counting – because in that case we would be stripping the day of the Holiness that we have already granted it. Adding that Holiness later, says the Ra’avad, is not a problem at all. (The Mishnah Berurah [Source 17] would agree, albeit for a different reason.)
3) See Sources 34-35 for a surprising debate between the Rambam and the Ritva as to whether it is in fact the case that all of Israel should keep one day and all of “non-Israel” should keep two days. The Rambam (Source 34) is of the opinion that locations in Israel which would have been more than a ten-day walk for the original messengers of Beit Din, or which were not settled at the time of the Beit Hamikdash, should keep two days, while areas outside of Israel which are within a ten-day walk (his example is Egypt) would in fact keep one day. The unassailable logic of the Rambam is that even preserving previous customs – הזהרו במנהג אבוביכם בידיכם (see ביצה דף ד) – would not indicate a need to go beyond those customs insofar as they ever would have been practiced. The Ritva (Source 35), however, feels that once the order was sent from Bavel to Israel that previous customs should be honored above calculations or calendar, the intention of the order was that all “non-Israelis” follow the majority custom of “non-Israel” (two days), and all Israel follow the majority custom of Israel (one day). (Think of ותן טל ומטר לברכה, for example, which we say based on the needs of Bavel rather than those of each individual country – even when, as in the case of South Africa, Bavel’s needs are entirely incompatible with the actual needs of that locale.)
In the course of a long-winded answer (most of which I have left out of the Sources) discussing the greatness of Hillel whose calendar served to preemptively witness all future new moons, the Ritva touches on something which may give a possible clue why all of Israel keeps one day while all Disapora keeps two. It is well known (particularly from Gemara Sanhedrin) that it was only in Israel that testimony or calculations to the new month (or leap year) could be made. Perhaps the contemporary divide between Israel and the Diaspora is not based on previous routes of messengers but on the fact that, in the absence of witnesses, we can only rely on calculations or a calendar to determine the correct date in conjunction with one’s being in a location in which those messengers would have been meaningful were there still a Beit Hamikdash today. Sure, Hillel’s calendar is strong enough to stand in place of witnesses until the end of time – if indeed those theoretical witnesses would have ever been accepted in one’s given location. If, however, witnesses never would have been accepted in your hometown anyway, the calendar cannot possibly serve as a meaningful replacement for those very witnesses.
4) Concerning the famous Talmudic rule (Source 14) that on Shemini Atzeret, מיתב יתבינן ברוכי לא מברכינן – we sit in the Succah without a Beracha, why don’t we also shake a Lulav without a Beracha on Shemini Atzeret? Someone suggested to me that sitting in a Succah is דאורייתא all seven days, while Lulav is only דרבנן after the first day. That’s a good answer, but it raises a different problem – the Torah proscribes sitting in the Succah for seven days (בסוכות תשבו שבעת ימים), which we have already done before Shemini Atzeret begins, so why keep going? Lulav is not given for a clear number of days in Chumash (ולקחתם לכם ביום הראשון), so perhaps one more day would not be harmful!
Anyway, those are a few things – ואידך זיל גמור.