Question: I know on seder night we must wait until nacht to begin the seder. Is it important to wait for nacht on the second night of other yamim tovim?
Answer: The Taz (OC 489:10) maintains that as the first day of yom tov has a higher level of kedusha than the second day, one should not begin the second day of yom tov before nacht. Nonetheless, the Mishna Berura (489:23; Shaar Hatzion 789:51) notes that the consensus of poskim is not to pasken this way.
The Mishna (Shabbos 113a) teaches that one must not prepare one’s bed on Shabbos for after Shabbos as it is a prohibition of hachana, preparing. This prohibition applies equally to yom tov. The Levush (OC 488:3) writes that one must wait until it is nacht to daven maariv and perform melacha on the second night of yom tov, so as not to perform hachana on the first day for the second. The Mateh Ephraim (599:2) adds that as people wait until maariv starts to perform melacha for the second day, it is imperative that they wait for nacht to begin maariv.
R’ Yitzchak Yaakov Weiss (Minchas Yitzchak 10:41) explains that the second nights of Shavuos, Rosh Hashana and Shemini Atzeres are different to the second night on Pesach and Sukkos as there is no specific obligation to eat matza or eat in the sukka that night. Ideally, one should wait until nacht on Shavuos.
However, the Ben Ish Chai (Bamidbar 1:2) writes that one may start the second day earlier, especially on Shavuos when nacht is so late. Ideally, one should light candles before reciting kiddush. Therefore, if one recites kiddush before nacht, there is no issue of hachana to light first.
In conclusion, it is ideal to wait till nacht to daven maariv on the second night of Shavuos, but if necessary, one can daven earlier.