On a festival it is permitted to cook and to perform certain labors for the sake of the festival itself, provided that one uses an existing fire rather than igniting a new flame. What is the halakha when the festival falls on a Friday and one wishes to cook on the festival for the sake of Shabbat?

It is permitted by Torah law to cook on the festival for Shabbat, but it is prohibited by rabbinic law unless one prepares an eiruv tavshilin before the festival. There are two reasons for this decree of the Sages: (1) To prevent people from mistakenly thinking that just as it is permitted to cook on the festival in honor of Shabbat, so too, it is permitted to cook on a festival for the weekdays that follow Shabbat; and (2) due to the concern that the great deal of cooking done for the festival might cause one to forget to set aside good-quality food for the next day, Shabbat.

Therefore, the Sages instituted the eiruv tavshilin, which has a dual purpose: (1) To serve as a reminder that it is prohibited to cook on a festival for the weekdays, as even in honor of Shabbat one may not cook on the festival without preparing an eiruv tavshilin; and (2) the eiruv tavshilin, which is preserved for Shabbat, reminds the members of the household to prepare fine food for Shabbat.

This section explains what an eiruv tavshilin is and how it is prepared.