Eruv Tavshilin by Rabbi Yair Hoffman

FiveTowns Jewish Times   In the Hebrew the numbers 5774 are expressed as Tof shin ayin dalet.  One could understand these letters as an abbreviation for the expression, “Tehye Shnas Dalet Eiruvin” it will be a year in which four Eruv Tavshilins will be made.”

What is an Eruv Tavshilin?

Whenever Yom Tov comes immediately before Shabbos, in other words, on a Friday – an Eruv Tavshilin must be made. An Eruv Tavshilim is a Rabbinic device that allows one to “continue preparing and cooking” for Shabbos on Yom Tov. In other words, through the Eruv Tavshilin, one actually begins the Shabbos preparations on the day before Yom Tov.  The Malachos performed on the Yom Tov for Shabbos are considered to be a continuation of these preparations. (Rema O.C. 527:1)

We learned, however, that performing Malacha on a Yom Tov for another day other than the Yom Tov itself is, in fact, a Torah prohibition.  If this is the case, then how could it be that a Rabbinical enactment allows one to get around a Biblical prohibition?

The answer is that, technically, it was permitted by the Torah to cook on Yom Tov for Shabbos.[...]