I'm guessing by your username that you're an AST. If you are.. hey, sister!
As far as I know, we do not have an official pref ceremony. When I was my chapter's Recruitment Advisor, the actives held a short "ceremony" on bid day (we only did informal at the time). It was a nice way to welcome the new members into the chapter and surprise them with their bids.
If I remember correctly, we found this ceremony (intended for pref) in the Recruitment Handbook which we accessed through the Officer Resources area of the national website. I believe there were a few optional ceremonies, but nothing official.
I would confirm this with an advisor, or with someone higher up. But I would think that if we had a mandatory ceremony, your chapter would have been informed.