According to this page http://www.shemayisrael.co.il/dafyomi2/sukah/insites/su-dt-44.htm
the idea that lo tisgodedu is applied to the lulav on first day of yomtov on shabbos is only according to Rashi, and was established at the time BEFORE there was a fixed calendar, thereby relating first day yomtov in chutz la'aretz to sfeka deyoma. Presumably besides Rashi, if there is no sfeka anymore, and taking the lulav is a d'oraisa, then we should have the ability in EY or in Chutz La'Aretz equally to take the lulav, to be machmir to bring the lulav on Friday to shul, or to take the lulav at home.
Also, if second day yomtov bizmanenu is just a custom of ancestors, then how can this minhag be doyche the d'oraisa of tefillin?! Thanks.
David Goldman, USA
1) The conclusion of the Gemara is that after the destruction of the Beis ha'Mikdash, even if the first day of Sukos falls on Shabbos, one does not take the Lulav even in Eretz Yisrael.
2) The Gemara (Beitzah 4b) states that even though nowadays we have a fixed calendar, nevertheless a teaching was sent from Eretz Yisrael to Bavel, "Be careful with the custom of your fathers!" and continue to observe two days of Yom Tov.
3) There is a rule in many places in Shas (see, for example, Pesachim 30b), "Wherever the Rabanan made a Takanah, they did it in a similar way to the Torah law."
The Rashba (Teshuvos 1:61) writes that according to this, since it is forbidden to wear Tefilin on the first day Yom Tov (because of "Bal Tosif"), it follows that when the Chachamim instituted the Din of second day Yom Tov, they said that it has the same status as the first day and therefore one may not wear Tefilin.
4) The Rashba explains that the reason why the Chachamim's Din is equated with the d'Oraisa Din is that, otherwise, "Asi l'Zilzulei Behu" -- "people will make light" of Yom Tov Sheni. Since Yom Tov Sheni is a custom, if one would allow wearing Tefilin, people would say that Yom Tov Sheni is not important.
5) The Rashba also writes that not wearing Tefilin on the second day of Yom Tov is considered "Shev v'Al Ta'aseh"; it is a passive action. The Gemara in Berachos 20a says that Shev v'Al Ta'aseh is different. Rashi (DH Shev) writes that Chazal permitted uprooting many things which are d'Oraisa in order to make a fence around the Halachah.
Thank you, R. Dovid Bloom. I just want to make the following points. The sources referred to in that page described second day yomtov and not taking lulav on first day/shabbos as "customs of our ancestors." This is not the same thing as an actual gezeyra of the Sanhedrin, because if it were, it would not be stam a minhag but a binding law, and is not even a minhag of Chazal themselves. Furthermore it only applies in the time before the fixed calendar as a binding law, but not to when we have a fixed calendar.
The requirement of the second day in Beitsa is a warning or an appeal, but is not a binding law (because the application was because of the Kusim), for which it would say so explicitly, and all the rishonim and achronim who refer to second day yomtov as a MINHAG are not saying it is a law. R. Chaim Brisker described it as a chumra (which would be "custom of our ancestors). So a gezeyra or takkana here is not the same as a mere custom of ancestors.
Finally, regarding points 4 and 5, I very humbly cannot understand the Rashba or Rashi, because a) even if there were a "zilzul" in second day yomtov by wearing tefillin, second day is STILL only a custom, but TEFILLIN are a D'oraisa! I also do not understand why he would say it is "shev ve'al taaseh" not to do a d'oraisa mitzva SIMPLY because of an ancestral custom! Rashi in reference to uprooting d'oraisa cannot be referring to second day yomtov (or even not taking lulav on shabbos first day) because these are ONLY minhagim and not rabbinical injunctions bizmanenu.
David, thank you once again for your extremely insightful comments.
1) Maseches Sofrim 14:18 states a very important principle: "Minhag Mevatel Halachah" -- "custom abolishes Halachah." The Magen Avraham (Orach Chayim 690:22) writes that when we say that custom uproots Halachah, this refers only to a custom which was founded by experts. We learn from this that a well-based custom receives the status of a binding law.
2) The Gemara itself in Beitzah 4b refers to the observance of the second day of Yom Tov outside Eretz Yisrael as a custom. As you correctly point out, it is not a Gezeirah of the Sanhedrin. However, since the Gemara there tells us that they sent a directive from Eretz Yisrael to be careful about this custom, this means that it is considered a custom with expert backing. Now we can understand how the custon can uproot the Torah Mitzvah of Tefilin on the second day of Yom Tov.
3) "Shev v'Al Ta'aseh" ("sit and do nothing") merely signifies that a Mitzvah is being transgressed in a passive, not an active, way. Certainly, Tefilin is an extremely important Mitzvah but since it is being uprooted on the second day of Yom Tov in a passive way, the custom of Yisrael posseses sufficient power to do this.
Does this therefore mean that the explanation of the Magen Avraham must be the binding view on this matter of second day yomtov? I recall that the Brisker Rav, R. Chaim, calling it a chumra nowadays did not invoke a statement from the Magen Avraham as an underlying bind view for keeping second day yomtov.
Yes second day Yomtov certainly is obligotary nowadays.