Im having elderly people over for Rosh hashana. Can I make Kiddush early after Plag on the first night, second night, and this year Friday night? Please answer asap. Thank you.
Yes, under the circumstances you can make Kiddush early on both nights.
It is possible to start Rosh Hashanah early (after plag), like Shabbos and Yom Tov generally.
The Vilna Gaon states (Maaseh Rav) that Kiddush must be made before davening. However, other authorities do not mention this concern, and the common custom is that where necessary one make Kiddush before davening.
Accepting the second day of Yom Tov early is the subject of a dispute among authorities, as mentioned in siman 489 (the Taz is stringent, and the Kesav Sofer and Chok Yaakov are lenient). The issue is whether accepting the second day early is a pegam (flaw) in the holiness of the first day.
The Minchas Yitzchak (1:42) thus writes that for somebody who is sick or elderly one can be lenient. See also Rivevos Efraim (8:459) who is lenient, but does not present any sources.
Some, however, argue that those who dispute the ruling of the Taz only do so after it is dark, and not before it is dark (see Maros Yesharim 7). Where possible, one should refrain from accepting Yom Tov Sheini early – but for elderly people and a need, it is permitted.