748. Neglecting to Make an Eiruv Tavshilin
102:5 The eiruv must remain intact until one has prepared all of his needs for Shabbos. If the bread was lost or eaten, it doesn't matter and one may still bake for Shabbos. If the cooked item was lost or eaten, one may still rely upon it so long as an olive-sized portion (k'zayis) still remains. If a k'zayis no longer remains, one may not cook on yom tov for Shabbos and it's as if he didn't make an eiruv at all.
What should one do if he forgot to make an eiruv tavshilin? If he didn't realize that he forgot until after the morning meal - and if there is not another Jew in the area who made an eiruv - then one may only cook one pot of food, bake one loaf of bread, and light one candle for Shabbos. If he realized that he forgot before cooking the morning meal, he may cook some of each type of food in a large pot and save it for Shabbos. If there is someone else around who did make an eiruv, he can give that person some of his flour, meat and other ingredients as a gift. The other person takes possession of these items by raising them up and he may then cook and bake for the one who forgot, even in the house of the one who didn't make an eiruv.
102:6 Every homeowner must make an eiruv tavshilin for himself. (Another person may make it on his behalf – Mishnah Brurah 527:25.) A woman with no husband is likewise obliged to make an eiruv. It is forbidden to intentionally refrain from making an eiruv, relying on the eiruv made by the leaders of the city. One who forgot to make an eiruv tavshilin due to extenuating circumstances, or if he made one and it was lost, then if there is someone in the city one who makes an eiruv for all the residents, he may rely on this eiruv. (A person can rely on the communal eiruv in such a case only once; he may not do so repeatedly – MB 527:22.) However, if he forgot out of laziness or if his intention at the outset was to rely on the communal eiruv, he may not benefit from this and must follow the procedures outlined in 102:5. (A person who does not know how to make an eiruv may rely on the communal eiruv – Rema 527:7.)