Shavuot

Eruv Tavshilin

Eruv Tavshilin is made when Shabbat begins immediately following Yom Tov. Cooking on Yom Tov is permitted for the needs of that day. The rabbis permitted cooking on Yom Tov for Shabbat provided that preparations for Shabbat begin before the holiday. Those preparations happen through eruv tavshilin.

Here's how:

Before Shavuot begins, bring together two prepared foods, usually a baked food like challah and a cooked food like a hard boiled egg or a piece of chicken or fish. Recite this blessing:
בָּרוּךְ אַתָּה יי אֱלֹהֵינוּ מֶלֶךְ הָעוֹלָם אֲשֶׁר קִדְּשָׁנוּ בְּמִצְוֹתָיו וְצִוָּנוּ עַל מִצְוַת עֵרוּב
Barukh ata adonai eloheinu melekh ha’olam asher kid’shanu b’mitzvotav v’tzivanu al mitzvat eruv
Blessed are you Holy Gd who rules the universe and sanctifies us through responsibilities and mitzvot like the practice of creating an eruv.
With this eruv we have permission to bake, cook, heat food, light candles and do what is necessary on the holiday in preparation for Shabbat.

Now set the eruv foods aside to be eaten on Shabbat during the day.