One may not prepare from Yom Tov to Shabbat unless one does Eruv Tavshilin before Yom Tov.
This year the 7th day of Pessach is on Friday, so one needs to do Eruv Tavshilin on Thursday-Chol-Hamoed.
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One takes 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:בַּהֲדֵין עֵרוּבָא
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 (from an existing flame.)
Source: Kitzur Shulchan Aruch 102
Tuesday, 11 Nissan 5775