An eiruv tavshilin is a symbolic preparation of food from the eve of the festival for Shabbat. It is called an eiruv, meaning mixture, because it serves to combine the food for the festival and Shabbat; the result is that just as it is permitted on the festival to cook and bake for the festival itself, so too, it is permitted to cook and bake on the festival for Shabbat.

Even one who is not planning to cook during the festival should prepare an eiruv tavshilin, as otherwise it is prohibited to perform any action on the festival for Shabbat, including lighting Shabbat candles toward the end of the festival.