When Yom Tov continues into Shabbat, an Eruv Tavshilin needs to be made before lighting Yom Tov candles.
What Is Eruv Tavshilin?
To preserve the holiness of Yom Tov, we cannot spend time on Yom Tov preparing for another day – a prohibition known as hachanah, even if that other day is another festive day like Shabbat or second day Yom Tov. For Yom Tovim that fall on a Thursday or Friday, Chazal instituted the making of Eruv Tavshilin as a way of pre-emptively combining food that has already been prepared for Shabbat before Yom Tov with food that we plan to cook on Yom Tov for Shabbat.
For the Eruv Tavshilin to be valid, the food being cooked on the Friday must be fully prepared well before the onset of Shabbat.
How To Make Eruv Tavshilin
Set aside a piece of baked food at least 30g in weight (such as a matzah) and an item of cooked food at least 15g in weight – the word “tavshilin” means cooked food (such as chicken or meat). Both foods should be food items that will be eaten on Shabbat and set aside accordingly.
Before Yom Tov, make the brachah and the declaration for the Eruv which can be found at the beginning of most Machzorim or in most Siddurim. It is important to understand what we are saying and so it is permissible to say it in English or another language we understand better than the original Aramaic.
How Does Eruv Tavshilin Work?
By combining food that has been cooked before Yom Tov with food we plan to cook on Friday/Yom Tov, which could theoretically still be eaten on Yom Tov if we wished, we merge them into a single halachic entity (the word “eruv” means to merge), and so, just as the food we prepared on Erev Yom Tov can be eaten on Shabbat, the food that will be prepared on the Friday/Yom Tov can also be eaten on Shabbat.