Although it is permitted to cook on Yom Tob, one may only cook for consumption on Yom Tob itself. There is a Rabbinic prohibition to cook on a Friday Yom Tob for the Shabbat. However, the Misva of Erub Tabshilin was instituted to enable one to cook food for Shabbat when Yom Tob falls on Friday.
The question was asked: Does Erub Tabshilin also allow one to cook from the Thursday of Yom Tob, or only from Friday?
This issue is discussed in Teshubot HaRosh (23:8) and Teshubot HaRashba (1:485). They conclude that the Rabbis "Only permitted a close infringement and not a far infringement." Meaning, Erub Tabshilin only allows one to cook from Friday, which is closer to Shabbat. Therefore, even if a woman is going to be very busy on Friday Yom Tob, the Erub does not allow her to cook for Shabbat on the Thursday of Yom Tob.
In the event she violated this Halacha and cooked for Shabbat on the Thursday of Yom Tob, Hacham Ovadia rules that the food is not forbidden to be consumed.
SUMMARY
Erub Tabshilin does not enable one to cook from Thursday Yom Tob for Shabbat, only from Friday.
