One additional solution may be available to one who has no ET to rely upon:
21) If it is still early in the day on Yom Tov/Friday, such that one has not yet eaten his main Yom Tov meal, the halacha is as lenient as possible with respect to the solution of ‘marbeh b’shiurim’ (i.e. technically cooking for Yom Tov, but making more than one needs so that much remains for Shabbos.) This leniency – more than permitted normally, as taught in MBY 503:1-2 - is as follows: Even if one doesn’t really need any more food cooked for Yom Tov, he can make even several pots of different kinds of food, as long as he tastes from each one at the Yom Tov meal. The rest of the food can be left over for Shabbos! Although it is obvious from the circumstances that he is doing this solely so that he can have food for Shabbos, it is permitted in light of the great mitzvah of ‘simchas Yom Tov” (Yom Tov joy!) If one has already eaten his Yom Tov meal, this solution is not available, and one will have to resort to one of the solutions taught in the previous lesson. (See #23-24 for how this kind of leniency can be abused…)
22) Here’s one last fall-back based on an amazing question: If a person forgot to make an ET on Erev Yom Tov (this year on Wednesday), could he actually make it on Thursday (first day of Yom Tov), so that he may cook on Friday for Shabbos?
Here’s an even more amazing answer (!): Technically, one cannot make an ET on Yom Tov itself. However…. why are there two days of Yom Tov anyway? Isn’t it because it was instituted as a permanent custom to act as in the days when the Diaspora Jews kept two days out of doubt as to when the true day of Yom Tov was? In other words, in those days, only one of the two days was actually Yom Tov; it’s just that since they didn’t know which, they observed both ‘mi-safek’ (out of doubt). So, hear this Rabbinic logic which will “knock your socks off”:
On Thursday, take some ET food in your hand and make the following declaration (without making the bracha):
“If today is the actual Yom Tov, I don’t really need an ET, because tomorrow is actually a weekday, and I can cook tomorrow for Shabbos without a problem. If tomorrow is the actual Yom Tov, today isn’t, and I can still make an ET (!), so: ‘Through this eruv may we be permitted to bake, cook…!’” Now he is covered, and either way, he may cook on Yom Tov/Friday for Shabbos! Isn’t that the absolute neatest thing you ever learned?!
Note: This procedure will not work if Thursday and Friday are the two days of Rosh Hashana! The two days of Rosh Hashana are not based on the “doubt” concept. Rather they were instituted as one long two-day holiday, which is why we eat a new fruit on the second night of Rosh Hashana, not on the other two-day Yomim Tovim. The reason for this distinction is way beyond the scope of this lesson, and will be revisited, iy”H, when we learn the Halachos of Rosh Hashana!
23-24) If a person, who did not have an ET to rely upon, violated the halacha and cooked on Yom Tov for Shabbos – either intentionally or un-knowingly – he may still eat the food after-the-fact on Shabbos. However, if someone made an outright cunning justification - e.g. he cooked an extra pot on Friday on the grounds that that “maybe guests will show up”, and then “decided” to use it for Shabbos (without eating any of it, as was taught in #21) - he is penalized and neither he nor the members of his household may eat from the food on Shabbos! The point is that the Sages were more wary of crooked halachic justifications spreading to others than they were of outright violation of their decrees.