For my Pref Night I wore a nice black dress, and so did everyone in the house I pledged AND most of the PNM's. It's not required to wear a black dress. Just wear what you feel comfortable in.
At my school Pref Night is just the name for the last day of parties. The Pref Ceremony is just a short little thing that the actives do to show the PNM's what the house is like. At my house we do a simple candle lighting ceremony and read poems.
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"We're not good because we're old. We're old because we're good!"