The Laws of Yom Tov part 2 vol.3 no.12
Transferring objects One may transfer objects from a private domain to a public domain for the needs of Yom Tov on Yom Tov. One is also allowed to carry in a public domain even in a city which does not have an eiruv. For example one may carry his siddur to shul on Yom Tov and if he requires it for any reason he can bring it back home. (The definitions of public and private domain are too detailed to fully explain in this issue.) One should refrain from carrying heavy loads in the same manner as he would during the week.
Eiruv tavshilin One is not allowed to prepare on Yom Tov for the needs of the next day.However, if the next day is Shabbos one can prepare for it if he sets aside an eiruv tavshilin before Yom Tov. An eiruv tavshilin consists of one cooked food and a bread product, preferably whole. Setting these foods aside is considered like having begun his preparations for Shabbos which can then be continued on Yom Tov. For practical purposes many people use a whole matzah and an egg for the eiruv tavshilin. It is customary to eat them during the third meal on Shabbos.
The afternoon before Yom Tov one holds the foods in his hands and recites the blessing Al Mitzvahs Eiruv and the declaration found in the siddur “With this eiruv we will be allowed to bake, cook, insulate, light candles, and do all our needs from Yom Tov for Shabbos.” (Needless to say this does not permit actions forbidden on Yom Tov.) This recitation can be said in any language he understands.
Someone who forgot to make or lost his eiruv tavshilin can rely on the eiruv tavshilin of the Rabbi of the city or someone who specifically sets aside an eiruv tavshilin for the sake of anyone that might need one. However, one may not refrain from making his own eiruv tavshilin having in mind to rely on this person’s eiruv tavshilin.
One should be careful to perform his preparations for Shabbos early in the afternoon on Yom Tov. More specifically, one should leave enough time for any possible guests to partake of the food on Yom Tov itself. However, even if one did not begin his preparations until later on in the day there are opinions that permit him to prepare for Shabbos.
This issue dedicated to the memory of Devorah Felstein of blessed memory.
