What big event is scheduled to take place the next day? What surprised Hannah most about this conversation?

The big event is Shmuel's wedding.

What surprised Hannah most about the conversation was that she understood what Gitl and Shmuel were saying..... they were speaking in Yiddish, and Hannah did not understand Yiddish.

Nothing they had said seemed at all funny, but that she'd understood them at all seemed miraculous. For the more they talked, the more she realized they were not talking in English. They were speaking Yiddish. And yet she could understand it, every word. Perhaps of all the strange things in the dream, this was the strangest.

This is a follow-up question what was Shmuel and Gitl conversation consist of in other words what were they talking about in Yiddish that Hannah understood?