One may not prepare from Yom Tov to Shabbat unless one does Eruv Tavshilin before Yom Tov.
Since Rosh Hashana is on Thursday & Friday, every household needs to do Eruv Tavshilin on Wednesday.
One takes bread or Matza and a cooked or baked food that will be eaten on Shabbat and one says the Bracha:
One continues with the following, which must be said in a language one understands:
"With this Eruv we are permitted to bake, cook, keep things warm and light fire and do all that is needed from Yom Tov to Shabbat".
The Matza and food should be kept in a safe place; it they are eaten before Shabbat, a Rabbi should be consulted how to proceed.
Even those who don't have to cook for Shabbat, still need to do Eruv Tavshilin in order to light Shabbat candles on Friday before sunset (from an existing flame).
Source: Kitzur Shulchan Aruch 102
Tuesday, 28 Elul 5770
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