An Eruv Tavshilin is a halachic device which allows one to cook food on the Festival for use on the Shabbat that immediately follows.

To allow one to cook on the Festival for use on Shabbat, one must prepare an eruv tavshilin which serves as a reminder that the cooking done on the Festival is for Shabbat only.

This eruv consists of a matzah and something cooked, for example, a boiled egg or a piece of fish, and is prepared before the onset of the Festival.

It symbolically serves as the beginning of the preparation of food for Shabbat. Thus, any subsequent cooking done on the Festival is considered to be a continuation of the preparation begun before the Festival.

In the Diaspora, if the first two days of Passover fall on a Thursday and Friday, one must prepare an eruv tavshilin on the afternoon before Passover so that one can cook on the Festival for Shabbat the first of the intermediate days.

It should be noted that in Israel, one need never prepare an eruv tavshilin on the day before Passover since the first day of Passover never falls on a Friday.

It is possible, however, for the last day of Passover to fall on a Friday. Thus, in Israel as well, there are times when one would have to prepare an eruv tavshilin on the last of the intermediate days.