A person has to eat in the sukkah after tzais hakochavim on the first 2 nights. If someone has an elderly mother or elderly guests,can one eat in the sukkah before tzais or earlier like the gra? Whats the earliest?
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.