פירוש על עבודה זרה 15:18

The Talmud asks a strong question—why has the Jew transgressed just because the idolater calls him to the meal? Why not say that he has transgressed only if he actually eats of the sacrifice? Rava reads this out of the verse, which says “and he will call you.” But I think that there is more than a midrash at work here. There is a deeper statement—even if the Jew does not actually transgress, the fact that he lives in an idolatrous environment, in a place where idolaters will be inviting him to participate in their celebratory meals, is already a ticket to transgression.

This piece outlines when a Jew can accept the invitation from the idolater. When do we assume that the invite was part of the wedding celebration, in which case it is prohibited? And when can we assume that it is safe? The prohibition of participating in the meal begins from the time that the barley was thrown into the tub, in order to make beer.
We should note that this same exact sugya appears in Ketubot concerning reciting the special wedding blessings (today called the sheva berachot) When do we recite the special blessings and when do we not? That was probably the original context of this material.