* The full details of this law is elaborated in the Sefer “The Laws and Customs of Erev Shabbos and Motzei Shabbos” chapter 5 regarding Erev Shabbos, and the same applies for Erev Yom Tov. Refer to there for further details and Q&A on the subject.

The general rule: It is a Mitzvah[2] to refrain from beginning a meal past the 10th Zmaniyos hour of Erev Yom Tov, just as is the law regarding Erev Shabbos.[3] [This is approximately three hours before sunset]. The above is only with regards to a set meal [i.e. 55 grams of bread[4]] however it is permitted to eat a mere snack up until sunset and there is no need to refrain from doing so.[5]

May one eat a meal past the 10th hour if he did not eat prior? If one transgressed or forgot and did not eat prior to the 10th hour, then he may eat a meal past the 10th hour [with exception to Erev Pesach and Erev Sukkos].

A large feast and Seudas Mitzvah:[6]  

It is forbidden for one to establish a large feast on Erev Yom Tov, which one is not accustomed to eat during the week, if the feast involves drinking alcoholic beverages. It is forbidden to eat such a feast even on Friday morning.

Seudas Mitzvah:[7] The above prohibition only applies with regards to an optional feast, or a Seudas Mitzvah which is optional in terms of the date in which one must set it for, and it is thus possible for one to delay it to a later date. However a Seudas Mitzvah which has fallen out to be eaten specifically on Erev Yom Tov may be set on Erev Yom Tov. Nevertheless even when the date of a Seudas Mitzvah falls on Erev Yom Tov one is to initially begin the meal prior to the 10th hour of the day[8] and he should only invite ten people besides for close relatives and the Baalei Hasimcha. The following is the list of Seudas Mitzvah occasions which may be celebrated on Erev Yom Tov:

Bris Mila [whether it being the 8th day after birth or past the 8th day after birth]; Wedding feast; Pidyon Haben [if it is taking place on its proper time which is the 31st day after birth].

 Summary:

On Erev Rosh Hashanah it is a Mitzvah to refrain from beginning a meal past the 10th hour of the day. Snacks may be eaten throughout the day even past the 10th hour. If one transgressed or forgot and did not eat prior to the 10th hour, then he may eat a meal past the 10th hour.

Seudas Mitzvah: It is forbidden to eat a large meal any time on Yom Tov unless it is a Seudas Mitzvah which its date has fallen on Erev Yom Tov. In such a case one is to initially begin the meal prior to the 10th hour of the day and should only invite ten people besides for close relatives and the Baalei Hasimcha.