Dear Rabbi, I’m having elderly people over for succos. When is the earliest one can make Kiddush and eat in Sukkah on the first and second night in Chutz La’aretz? Please also explain second night regarding this halacha?
For the first night, you should still wait until nightfall before making Kiddush, though for the time of nightfall you can rely on the lenient opinions. For the second day, you can make Kiddush after shekiyas ha-chamah, after which there is already a doubt concerning nightfall (bein ha-shemashos).
The Magen Avraham (639:11, based on Rema) and the Shulchan Aruch Ha-Rav (639:2) explain that not only must a person eat bread after nightfall on Sukkos, but also that it is forbidden to eat before nightfall.
Therefore, one should ensure that one does not eat before nightfall (tzeis).
However, for the purpose of this prohibition, one can rely on the earlier time of the Gra, though one should ensure that he continues eating bread in the sukkah later in the night.
For extensive sources, see Chazon Ovadyah 1:1; Yabia Omer 10, Orach Chaim 47.
Concerning the second day of Yom Tov, which is rabbinic, one can rely on the doubt of bein ha-shemashos and make Kiddush after sunset.