Questions: I know that we do not normally daven maariv on Shavuos night until nacht. As we cannot daven in shul this year, can we not bring Yom Tov in early?
Answer: The Shelah (Shavuos 1) writes that one must not recite kiddush or begin eating one’s Yom Tov meal on the first night of Shavuos until it is nacht, nightfall. As the Torah writes that one must count seven complete weeks of the sefira, bringing Yom Tov early would diminish this. Just as the Gemara (Berachos 27b) teaches that one can daven maariv before Shabbos ends, so too, one can daven before nacht¸ though one must wait to recite kiddush. The Magen Avraham (494:1) and Pri Chadash (OC 494:1) pasken this way, too.
The Yosef Ometz (850) notes that he never witnessed this in Germany. Additionally, by doing so, it takes away from how much one can learn on Shavuos night. Likewise, the Korban Nesanel (Pesachim 10:2) writes that one does not need to wait until nacht to recite kiddush and begin their meal.
The Mishna Berura (261:19; Biur Halacha 261) writes that there is a machlokes as to whether the mitzva of tosefes Shabbos, to add on a little bit of time both at the beginning and at the end of Shabbos (Rosh Hashana 9a; Yuma 81b) is miderabanan or mideoraisa. The Avnei Nezer (OC 316:12) writes that the mitzva is mideoraisa and applies equally to bringing Yom Tov in early. Thus, R’ Yaakov Emden (Siddur Yaavetz, Shavuos 4) writes that in order to fulfil tosefes Yom Tov one should daven maariv early.
Nonetheless, the Taz (OC 494:1), Shulchan Aruch Harav (OC 494:2) and the Mishna Berura (494:1) write that one shouldn’t even daven maariv before nacht.
R’ Naftali Tzvi Yehuda Berlin (Haamek Davar, Vayikra 23:21) explains that the reason for waiting until nacht is because the Torah writes that Shavuos should be observed on that same day.
In conclusion, it is important to wait until nacht to begin Yom Tov and daven on Shavuos.