747. Limits of an Eiruv Tavshilin

102:3 The eiruv tavshilin is only effective to enable one to do what is needed for Shabbos on yom tov while there are still daylight hours remaining. This is because, if he had guests who had not eaten that day, they would be able to eat and enjoy yom tov with him during the day before twilight thanks to the work that he did that day. However, if there isn't enough daytime remaining to enjoy the fruits of his labors that same day, then the eiruv tavshilin is not effective. (One may act leniently in compelling circumstances – Bi’ur Halacha 527:1 s.v. v’al y’dei eiruv.) Therefore, the practice when yom tov falls on Friday is to recite the Shabbos evening prayers while it is still daytime, so that they will be able to finish preparations before one reaches "Mizmor shir l'yom haShabbos" (Psalm 92). The food that one keeps warm for Shabbos must start being heated while there are still daytime hours remaining in order to be at least one-third cooked (and therefore edible) before twilight.

102:4 One may only bake and cook by means of the eiruv tavshilin on Friday. If yom tov is Thursday and Friday, one may not cook or bake on Thursday for Shabbos. (One may certainly not prepare on yom tov for Shabbos if there is a weekday in between them – Mishnah Brurah 127:41.)