This year Shavuos came out Motzei Shabbos. The following shailo was raised. In regards to Shavuos and to other yamim tovim, may one be “makabel yom tov” early while it’s still Shabbos, make kiddush and begin to eat the seudah?
One should not make early kiddush on Shabbos, unless there is a great need, for instance for the sake of the elderly or sick.
Under these circumstances, kiddush can be made early, including havdalah, but without the berachah on the candle, and without performing other melachot. The berachah on the candle should be made later, after the time of motza’ei Shabbos.
For Shavuos, some write that one should not make kiddush early because of the idea of temimos, but others are lenient.
The Peri Megadim (Noam Megadim 20) writes that because the holiness of Shabbos is greater than that of Yom Tov, it follows that one should not daven until after Shabbos, in order to add on from chol to kodesh — just like on every motza’ei Shabbos. This is also ruled by the Tosefes Shabbos (288:9).
It is possible that this is meant on a level of lechatchilah, meaning that it is better to add on kedusha to Shabbos than to Yom Tov.
See also Minchas Shlomo (1:3), who is in doubt as to this halachah, and in Shemiras Shabbos Kehilchasa (47 note 111) Rav Shlomo Zalman is quoted as stating that one can be lenient for extenuating circumstances. See also at length the sefer Nasan Piryo of Rav Nasan (on Pesachim, 103a), who expounds on this question at some length, and explains that it is contingent on the nature of Tosefes Shabbos.
[Note that when Yom Tov falls on Friday, it is fine to make kiddush early while it is still Yom Tov.]
The Peri Chadash (494) writes, citing from Masas Binyamin, that one should not make kiddush early on Shavuos, and we have already discussed the issue here.
The Kaf Ha-Chaim (639:7) rules that this applies even to Yom Tov Sheini, out of concern that they should be switched, but the Minchas Yitzchak (10:41) rules that one can be lenient in this matter, for the reason that the sefirah has already terminated.