When a Shabbat Sheva Brachot takes place at Third Meal time, a problem arises in case it runs past shkiah (sunset). At a regular Third Meal, which traditionally ends after shkiah, the wine of the cup of bentching may not be drunk before Havdalah, and is used instead after Maariv for making Havdalah. At a Sheva Brachot, two cups of wine are poured: one is used for bentching and the other cup of wine is used for Sheva Brachot. There is a difference of opinion among poskim whether it is permissible to drink that wine after shkiah.
In order to avoid the halachic quandary and differences of opinion, it is advisable to conclude the Sheva Brachot before sunset. However, in case it does run past shkiah, the prevalent custom is to recite the Sheva Brachot without the blessing of hagafen, and the groom and bride drink from both cups of wine after Havdalah. There are also other halachic opinions put forth.
Rav Yosef Yeshaya Braun, shlita, member of the Badatz of Crown Heights, One Minute Halachah #311, reprinted from crownheightsconnect.com
