Mishnah Megillah 1:5 nicely sums up a major theme of this tractate:
The only difference between Shabbat and a festival is human sustenance.
All laws from the Torah and rabbis apply equally to both Shabbat and festivals except for preparing food. On festivals, the laws may be bent to allow food preparation — to increase the joy of the festival.
Today’s page deals with a very specific question: What if a festival falls the day before Shabbat? May one use the cooking exemption of the festival to prepare food for Shabbat? This is what the mishnah on today’s page of Beitzah addresses:
When a festival occurs on a Friday (Shabbat eve), one may not cook on the festival with the intent to cook for Shabbat. However, one may cook for the festival itself, and if there is leftover food, one may eat it on Shabbat. One may prepare a cooked dish for Shabbat on the eve of the festival and rely on it for Shabbat.
If Shabbat follows a festival, one might be tempted to cook for Shabbat on the festival because cooking, after all, is permitted on that day (and not on Shabbat). However, this is not allowed. Food that is cooked on a festival should be cooked purely for the purpose of enjoying it on that festival — and not for the sake of eating on Shabbat.
As is so often the case, though, intent matters. If you cook food on a festival that you intend to eat on that festival, but happen to have some leftovers, those may be eaten the next day on Shabbat. Because the food was cooked for the festival, nothing improper has gone into its preparation.
So no, one may not plan to “accidentally” over-cook on a festival. Which leaves us with the question: If Shabbat immediately follows a festival, what should you do about food on Shabbat? The answer is that you must prepare your Shabbat food on the eve of the festival — or at least you must begin to. This food cooked two days before Shabbat is called eruv tavshilin.
The rabbis understand that the dish cooked two days before Shabbat, the eruv tavshilin, will not constitute all the food eaten on Shabbat, but can then be mixed with food prepared on the festival for Shabbat and it will be as if all the food was prepared ahead of the festival for Shabbat. In the mishnah, Hillel and Shammai debate the minimal amount of food that can be used for this purpose:
Beit Shammai say two dishes. Beit Hillel say one dish. And they agree that a fish with an egg on it counts as two dishes.
The eruv, as those who joined us for Tractate Eruvin may remember, is a fascinating rabbinicdevice.The word literally means to merge or mix — one area into another (the theme of Tractate Eruvin) or one day into another. In this case, the eruv tavshilin is a bit of food that is mixed with other food to create the legal fiction that the food for Shabbat was not cooked on a festival.
There are several other and better-known examples of eruv. The Torah forbids leaving one’s habitation on the Shabbat (Exodus 12:22), but the rabbis decided that this was too restrictive and so they created the eruv hazerot, “mixing courtyards,” to theoretically merge private properties and enable neighbors to carry into each other’s homes. The eruv techumin is another example that enables people to walk beyond the city bounds on a Shabbat, as forbidden by the Torah. All of this was to make the festival or Shabbat an enjoyable experience without sacrificing the principles of the Torah. Of course, it helped that people lived in closed communities or small settlements. But the device is now used to enable observant Jews carrying across large cities.
Likewise, eruv tavshilin makes it possible to do a little cooking for Shabbat on a festival that comes immediately before, easing the burden of managing two adjacent sacred times.
Read all of Beitzah 15 on Sefaria.