From a clothing standpoint (or any other for that matter) there is no difference between formal and semi-formal night. Most people dress up for both. A few people don't dress up for either.
If it is not one of the "theme nights," most people treat Lumiere's just like any other restaurant--summer dress or skirt for ladies, dockers and polo shirt or sports jacket for men. Since there are the 2 dressy nights, no one really makes a big deal out of Lumiere's on other nights. THe "dressy restaurant" thing is a bigger deal on cruises that don't have a formal night (can you think of a sister ship?)