This year (5779), the seventh day of Pesach will fall out on Thursday night and Friday. Our Sages prohibited cooking on Yom Tov for Shabbat without placing an Eruv Tavshilin, which is designating matza and a cooked food on Erev Yom Tov (Thursday) for to be eaten on Shabbat in order for it to be recognizable that one is not beginning to cook or bake on Yom Tov for Shabbat; rather, one is merely concluding this preparation.
One needs to take matza and a hard-boiled egg on Erev Yom Tov (Thursday), and recites:
Baruch Ata Hashem Elokeinu Melech Ha’Olam Asher Kideshanu Be’mitzvotav Ve’tzivanu Al Mitzvat Eruv.
After reciting the blessing, one should say:
With this Eruv, we shall be permitted to bake, cook, light candles, and do anything necessary on Yom Tov for Shabbat
One then saves the matza and cooked food to eat them on Shabbat.