1,012. The Two Days of Yom Tov Outside of Israel

Shvisas Yom Tov 1:20

Just as it is prohibited to eat an egg laid on yom tov, it likewise may not be handled. If it becomes mixed up with even 1,000 other eggs, they are all prohibited on yom tov. This is because they will all become permitted the next day and a prohibited item that will ultimately become permitted is never negated, not even when mixed with thousands upon thousands of similar items. If a person slaughters a chicken on yom tov and finds inside it eggs that already have their shells, they are permitted. This is because this is not a common occurrence and the Sages did not enact decrees against uncommon occurrences that only happen infrequently.

Shvisas Yom Tov 1:21

The practice to observe two days of yom tov outside of Israel is a minhag. [“Minhag” is popularly translated as “custom” but this is not to imply that it is optional. Minhag is a form of halacha.] The second day of yom tov is a Rabbinic enactment created during the exile. Those who live in Israel only observe two days of yom tov on Rosh Hashana; this will be explained in Hilchos Kiddush HaChodesh.