The Shulchan Aruch O.C. 261:4 says that an eiruv may not be made once one was mekabeil Shabbos. Yet, in O.C. 393:2 the Shulchan Aruch quotes two views on the question. Why does the Beis Yosef not cite both views in siman 261?
The Dagul m’Revava answers that these simanim are speaking of two different cases. In siman 393 the Beis Yosef refers to making an eiruv during “tosefes shabbos.” In siman 261 he refers to making the eiruv after one has accepted Shabbos by saying barchu. The difference is as follows: tosefes shabbos means simply stopping work in advance of Shabbos, a pure issur gavra of doing melacha. Kabbolos Shabbos means accepting the kedushas hayom of the upcoming day. When speaking of the time period of tosefes Shabbos, when one has not yet accepted the kedushas hayom but has simply accepted the issur of melacha, the beis Yosef is willing to entertain two views as to whether an eiruv can be placed. Not so where one has accepted kedushas hayom of Shabbos, in which case placing an eiruv is certainly prohibited.
This Dagul m’Revava may shed light on a number of sources that relate to tosefes Shabbos that we discussed in the past. For example, Tosfos (Kesubos 47) holds that the din of ain m’arvin simcha b’simcha does not apply during tosefes Shabbos and a wedding may be held so long as it is not yet actual Yom Tov or Shabbos. Perhaps the logic here is that ain m’arvin simcha b’simcha applies to the kedushas hayom of shabbos or yom tov, but has nothing to do with the pure issr gavra created by tosefes.
The Netziv writes that the words “b’etzem hayom ha’zeh” used to describe Shavuos tell us that there is no din of tosefes yom tov. Others go so far as to suggest that woman not light their Yom Tov candles early, as this would constitute an acceptance of the chag and cut short sefirah. Rav Shternbruch (Shu”T Tshuvos v’Hanhagos vol 4 #111) rejects this sevara. Even if we cannot extend kedushas yom tov earlier than nightfall, as we learned from the Dagul m’Revava, it is possible to have tosefes without making a statement about the kedushas hayom. Rav Shternbruch writes based on this that when one accepts tosefes on shavuos (the mitzvah of being mosif on shabbos or yom tov should ideally be done through a verbal declaration, not simply by stopping work) one should be careful to accept “tosefes yom tov” and not say that one is accepting the “kedushas yom tov.”
My son was very unhappy with this chiluk of the Dagul m’Revava (he’s in good company: the Aruch haShulchan writes that it is a distinction without a difference) and added a good point regarding the practical upshot. Neicha those who read my blog and are informed about such matters, but what of the rest of the world? As my son put it, what do I think his sisters have in mind when they light candles – do they think their kabbalah is merely tosefes, or do they think it is an acceptance of the kedushas hayom, like any other Shabbos or yom tov? (The way around that is to educate them about the sugya, which is sometimes easier said than done.)
On a final note, the practice of not saying kiddush until dark on Shavuos night (the Taz is the one who mentions not davening ma’ariv until dark, but most poskim refer to kiddush only) is very difficult to understand. Assuming the Dagul m’Revava is right, why should Shavuos be different than any other week when those who make early Shabbos recite kiddush during the period of tosefes even though the kedushas hayom of Shabbos is yet to arrive? M’ikkar hadin it seems there is no real issue here (Shu”T l’Horos Noson 7:31) and one would certainly be yotzei kiddush if it is recited early.