This is an important question. I will rephrase it slightly. Since the Gemara in Beitzah (2b) states that Yom Tov cannot prepare for Shabbos, which Rashi (DH v'Heichinu) explains to mean that the Torah placed importance on the Shabbos meal and required that it should be prepared before Shabbos on a weekday, how could the Rabanan institute the Halachah of Eruv Tavshilin which permits us to cook for Shabbos on Yom Tov which falls on Friday?

1. This question is asked by Tosfos (DH v'Hayah) who is perplexed that since "Hachanah" is d'Oraisa, how can we bake and cook on Yom Tov for Shabbos? Even if one says that the Eruv Tavshilin permits it, how can the Rabanan make a Takanah (Eruv Tavshilin) which uproots the d'Oraisa law?

2. Tosfos answers that Rabah, who says here that Hachanah is d'Oraisa, is consistent with his opinion in Pesachim (46b) where he maintains that if one bakes on Yom Tov with intent to eat the item on a weekday, he is exempt, because guests might arrive before the end of Yom Tov who will be able to eat the food. Similarly, one is permitted mid'Oraisa to cook for Shabbos on Yom Tov which falls on Friday because guests might come on Friday afternoon. Even though mid'Rabanan we do not rely on the possibility of the arrival of guests, since mid'Oraisa the cooking is permitted the Takanah d'Rabanan of Eruv Tavshilin has the power to permit cooking on Yom Tov for Shabbos, l'Chatchilah.

3. Tosfos adds that even according "to us" (Tosfos is implying that we do not follow the view of Rabah, but instead we rule that if one cooked on Yom Tov for a weekday he transgressed a Torah prohibition), one can understand how Eruv Tavshilin works. Baking and cooking are not actually defined as "Hachanah." The Gemara here is discussing an egg which was laid on Yom Tov. This egg is considered a totally new entity which was not in the world before Yom Tov. Therefore, Rabah maintains that the egg is not considered to have been "prepared" before Yom Tov. In contrast, when one cooks and bakes food, he is not considered as though he is creating something new. The food was fit for use before the cooking and one merely is improving it, so this act is not defined as "Hachanah."

Kol Tuv,

Dovid Bloom

The Mishneh Berurah (527:3) writes that when Yom Tov falls on Friday, one should be careful to prepare the Shabbos food early so that he can complete the work a good while before nightfall. This is because shortly before dark, at a time when it is not possible that the food will be needed on Yom Tov, there is a d'Oraisa prohibition against cooking. (Since it is not possible that guests will arrive so late, it emerges that one is certainly cooking on Yom Tov for Shabbos.)

Therefore, even though we say in the Nusach of Eruv Tavshilin that it permits us "to bake and cook... on Yom Tov for Shabbos," this does not mean that it is permitted for the entire day, but rather it is permitted only early enough on Friday afternoon that it is possible that potential guests who arrive could eat the food cooked.

Kol Tuv,

Dovid Bloom

1. When Tosfos (2b) writes that baking or cooking is not considered Hachanah he does not mean that baking and cooking are not important and significant activities. Rather, Tosfos is saying that the act is considered Hachanah mid'Oraisa only when one prepares something that did not exist before Yom Tov. An egg which had not been laid before Yom Tov is considered something new which came into the world on Yom Tov, while raw food, in contrast, did exist before it was cooked. Even though people might never eat the raw food before it is cooked, this still does not mean that one created the food when one cooked it.

2. It seems to me that the question about whether the Halachah changes nowadays that people do not eat raw chicken, etc., is more relevant when asked in a slightly different context: Is raw meat Muktzeh nowadays? The Shulchan Aruch (OC 308:31) rules that raw meat, even when entirely unsalted, is not Muktzeh on Shabbos because there are some people who chew raw meat. (Once it has been rinsed, there is no prohibition against eating it because of the blood inside it.) Is this also permitted nowadays when eating raw meat is surely unusual?

3. Tosfos writes only that Hachanah is applicable for an egg. However, there are one or two other examples. See the Gemara in Eruvin (38a) which states that one may not make an Eruv Techumin (to enable one to walk an additional 2000 Amos beyond the Shabbos boundary) on Yom Tov which falls on Friday. Rebbi Eliezer explains that the reason that one may not do this is because of Hachanah. By making the Eruv on Yom Tov one creates for himself a new Heter, which did not exist previously, to walk 2000 Amos beyond his original Techum. Tosfos there (DH Mishum) writes that the prohibition of Hachanah is mid'Oraisa.

4. The Tosfos ha'Rosh in Beitzah (2b, DH d'Amar Rabah) writes that the falling of the Man was also considered Hachanah. (In other words, if the Man would have fallen in the wilderness on Shabbos, Bnei Yisrael would not have been allowed to eat it because it was lacking Hachanah from before Shabbos.)

5. See Sefer ha'Chinuch (Mitzvah 298:6) who writes, "In my opinion, when one looks carefully in the Gemara, the only d'Oraisa prohibition of Hachanah that one finds is the prohibition of the egg." Nevertheless, we have shown that there are other Rishonim who do give other examples.

Kol Tuv.

Dovid Bloom