Q. On the first night of Shavuos, we wait to daven maariv until tzeis, (night), so the days of Sefiras Haomer will be complete. What about Tosefes Yom Tov, are we not supposed to add before every Shabbos and Yom Tov some time before sunset?
A. Mishna Berura (494: 1) quoting Taz indeed mentions to wait until night to daven Maariv. Similarly the Magen Avrohom rules in regard to Kiddush.
Many Poskim agree that Tosefes Yom Tov Biblically applies equally to all Yomim Tovim including Shavuos, (See Rosh Hashana 9a), which indeed creates a contradiction with the temimos requirement mentioned.
However Binyan Shlomo (Tikunim 20) debates whether Tosefes does apply or not to Shavuos.
Moadim Uzemanim (7: 233) suggests that there are differences between the Tosefes of Shavuos and the other Yomim Tovim, since Shavuos does not depend on a fixed date, but on the passage of the seven weeks.
Aruch Hashulchan (494: 3), Ben Ish Chai (Bamidbar), and others seem to say that the Temimos requirement applies only to Kiddush but not to Maariv.
Oz Nidberu, Hilchos Sefiras Haomer (11: 10) and others maintain that there are two kinds of acceptance of Yom Tov. One receives the complete kedusha of the day, while a limited Tosefes may be only the acceptance of not doing any more melochos or prohibited work, still, you comply with that the mitzva of Tosefes.
Halichos Shlomo (12: 9) and others are of the opinion that with the lighting of the Yom Tov candles, which is done when it is still day, one complies with the mitzva of Tosefes Yom Tov.
Horav Shlomo Miller’s Shlit’a opinion is that one complies with Tosefes Yom Tov by the acceptance of not doing any more melochos or prohibited work.
Rabbi A. Bartfeld as advised by Horav Shlomo Miller Shlit’a