A fundamental difference between Yom Tov observance and Shabbos observance is the allowance of ochel nefesh, food preparation on Yom Tov. “Ach Asher Yei’achel L’Chol Nefesh Hu Levado Yei’aseh Lachem…“1 The Torah permits us to cook, bake, and prepare food on Yom Tov proper, in order to eat the prepared food on that day of Yom Tov. One is not permitted to prepare from one day of Yom Tov for the second day of Yom Tov or for after Yom Tov. This prohibition of hachana, of preparing from one day of Yom Tov to the next, presents a problem when the second day of Yom Tov falls out on Shabbos or when Shabbos follows a two day sequence of Yomim Tovim. Can one halachically prepare food on Yom Tov for the Shabbos Yom Tov or for Shabbos?
To deal with this issue our Rabbis instituted a procedure known as eruv tavshilin2. An Eruv Tavshilin is prepared on Wednesday, if Yom Tov occurs on Thursday and Friday. It is prepared on Thursday, if the first day of Yom Tov is on Friday. The procedure is as follows. On Erev Yom Tov (Wednesday or Thursday), the head of the household, or his designee, should set aside a baked item such as bread or matzoh, and a cooked item such as meat, fish, or eggs (i.e. a food that is eaten along with bread). Each item should be at least the size of one kezayis, preferably the size of one beitzah. He or she should then recite the blessing of “Baruch…Al Mitzvas Eruv” and the proclamation, both found in the Siddur. This proclamation states that the cooked and baked items should permit us to continue baking, cooking, lighting a flame from an existing fire and do all the necessary preparations from Yom Tov proper to Shabbos. It is now viewed as though meal preparations for Shabbos have already begun before Yom Tov and Shabbos meal preparations may continue on Friday Yom Tov, Erev Shabbos.
Once done, the eruv covers all household members and guests.
The foods set aside for the eruv should be saved and may be eaten on Shabbos.
If one forgot to make an eruv tavshilin one should consult a competent Rabbinical authority for further instructions.