1,055. Eiruvei Tavshilin

Shvisas Yom Tov 6:1

When yom tov falls on a Friday, we may not bake or cook food to be eaten on Shabbos. This is a rabbinic prohibition, enacted to prevent people preparing food on yom tov for weekdays. This way, people will realize that if they can’t cook on yom tov for Shabbos, they certainly can’t do so for a weekday. Therefore, if a person prepares a portion of food on the day before yom tov and relies on it, he is permitted to cook and bake on yom tov for Shabbos. This portion of food that one prepares before yom tov and relies upon is called an eiruv tavshilin.

Shvisas Yom Tov 6:2

Here’s why it’s called an eiruv: the kind of eiruv that we make before Shabbos to enable carrying in courtyards and alleys draws a distinction so that people will not think that it is permitted to transfer articles between different types of domain on Shabbos. Similarly, this portion of food draws a distinction and reminds people that it is not permitted to make food on yom tov that will not be eaten the same day. For this reason, this portion of food is called an eiruv tavshilin.