Of course, there is ! Those are not the same word : specifically du is the contraction of de le.

In that specific case “sac de jour” means “day bag” as in a bag you would wear during the day and not in the evening (though it sounds a bit silly, then again : fashion), whereas a “sac du jour” would be a “bag of the day”, as in either the one of your many bags that you would wear specifically today, or a bag that has been made today (again, unlikely).