Again, be sure to wear comfortable shoes. Preference night will be the first night that you do not go back to all three sororities. This night you will be attending two parties - If you are invited back to all three, you will be choosing which two sororities to join for their parties. These events will be longer and the atmosphere is serious. For Preference night the dress should be a little more formal than on the other days.
Good choice is a dressy church dress.
On Bid Day you will find out the sorority that you will be a member of. Plan to spend the day with your sorority. They usually have a planned event where you will spend the day learning about your sorority and your new sisters. You will receive a Bid Day t-shirt from your sorority sisters and will place it over the shirt that you are wearing.