Wow, that is an early wedding! I would wear something neutral so you're not detracting from the color of the dress. Tan or taupe would probably be good choices. If you're leaning more toward white I would go with a cream or off-white color. White seems a little to harsh for purple. Have a wonderful time!
Depending on your age and the formality of the event, you could wear anything from a pair of ballet flats to a pair of dressy sandals to a pair of heels.