Having to find a white dress when it's not summer should be considered a form of hazing
My first initiation (into my local sorority) was a surprise... we only had a couple of hours' notice. It was October. We were told to wear all white or as close as possible. I had a white blouse, and I think I managed to dig up a white or light-colored skirt from among my summer clothes.
My initiation into AEPhi was in April, so some white dresses were already available.
Alpha Phi at my school wears white at pref, so between pref, pledging, and initiation, they get a lot of mileage out of their dresses