Halacha forbids cooking from Yom Tov to Shabbat unless one had prepared an Eruv Tavshilin before Yom Tov. If Yom Tov falls on Friday, then one prepares the Eruv Tavshilin on Thursday; if Yom Tov falls on Thursday and Friday, then one prepares the Eruv Tavshilin on Wednesday.
Of what should the Eruv Tavshilin consist?
Strictly speaking, it suffices to prepare a single cooked food; customarily, a boiled egg is used for this purpose. Although preparing one cooked item suffices to allow both cooking and baking on Yom Tov for Shabbat (Shulchan Aruch 527:2), nevertheless, the custom is to include bread in the Eruv Tavshilin, as well.
What quantity of food is required for the Eruv Tavshilin?
Chacham Bentzion Abba Shaul (Jerusalem, 1924-1998), in his work Or Le'tziyon (vol. 3, 22:1), writes that the cooked food should have a volume of a Ke'zayit – or approximately 1 oz. – and the bread should have a volume of a Ke'beitza – approximately 2 oz.
Chacham Bentzion Abba Shaul adds that the foods should preferably have been cooked and baked specifically on Erev Yom Tov, and not earlier. Nevertheless, one who set aside foods that were prepared earlier has satisfied the requirement and is allowed to cook on Yom Tov in preparation for Shabbat.
Summary: The Eruv Tavshilin, which one must set aside before Yom Tov to allow cooking on Yom Tov for Shabbat, should consist of 1 oz. of cooked food – customarily boiled eggs are used for this purpose – and 2 oz. of bread. Preferably, the foods should be prepared specifically on Erev Yom Tov, and not earlier.