The Laws of Rosh Hashanah vol. 4, no. 27
Selichos: According to the Ashkenazic custom selichos are recited starting the Sunday before Rosh Hashanah (this year coinciding with Septemebr 28). Since the selichos service is generally recited very early in the morning, one should be careful that he does not recite the blessing on the talis before the proper time.
Annulling vows: There is a long standing custom to perform the service of annulling one’s vows the morning before Rosh Hashanah (this year coinciding with October 1). The service can be found in most siddurim.
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 (as it will fall out after the second day of Rosh Hashanah) 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.
