Q. Regarding what Rav Miller said, to start the (Shabbat) day seuda before chatzos, does this same apply to Yom Tov? What about Rosh Hashana?
A. Horav Shlomo Miller’s Shlit’a opinion is that it also applies to Yom Tov, and to Rosh Hashana, even when it does not coincide with Shabbos, and an effort should be done to start the seuda before chatzos if possible.
The Rov pointed out that in regard to Rosh Hashana not on Shabbos, Poskim disagree if one is permitted to fast. Shulchan Aruch (O.H. 597 :1), rules that one is not allowed to fast, while Remo (ibid. 3) also quotes the opinion of those who on the contrary, maintain that it is a mitzva to fast. However, Mishna Berura (ibid. 12) and others assert that the Halacha follows Shulchan Aruch.
As pointed out on question 2062 one may drink something before davening so he will not be fasting on Yom Tov.
Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit’a