This year, the second day of Rosh Hashanah falls on Friday, followed immediately by Shabbat.
We are not permitted to prepare foods on the first day of Yom Tov for the second day of Yom Tov, or on either day of Yom Tov for the day after. However, when Yom Tov is followed immediately by Shabbat and it is necessary to prepare our Shabbat meals on Yom Tov, this may be done on the second day of Yom Tov (Friday) if we have made an Eruv Tavshilin.
It is the function of the Eruv Tavshilin to indicate that the preparation for Shabbat actually began before Yom Tov commenced. Food is set aside before the holiday begins (in this case, Wednesday afternoon) and is then eaten on Shabbat (usually for Seudah Shlishit). The preparation that we do on Yom Tov is then simply a continuation of what we began before Yom Tov.
The ritual of Eruv Tavshilin is found in most siddurim and machzorim. It is simple and is performed as follows:
On Erev Rosh Hashanah (Wednesday afternoon), before Yom Tov begins, take a challah or a matzah with a cooked food (such as an egg, a piece of chicken, or fish etc) and set it aside to be eaten on Shabbat. While holding the foods, recite the Bracha:
Baruch atah ad-nai, eloh-nu melech ha’olam, asher kideshanu bemitzvotav ve’tzivanu al mitzvat eruv.
(Blessed are You L-rd our G-d King of the universe who has sanctified us with His commandments and commanded us in the mitzvah of eruv.)
We then recite the following Aramaic declaration:
“Bahadayn aruva yehay sharay lanah la’aphoyay ulevashoolay uleatemoonay oleadelookay sheragah uletakana uleme’ebad kol tzarecanah, miyomah tavah leshabetah”
It is important to understand what you are saying, so if the Aramaic is Greek to you, the following English translation is also recited:
“With this Eruv, it will be permitted for us to bake, cook, insulate, kindle flame, and do all necessary things on Yom Tov for the sake of Shabbat.”
That’s it. Enjoy.
With best wishes for a Shana Tovah.