According to the Carthay Circle website, lunch runs through 3:30 pm and dinner starts at 4:30 pm. This means that the last reservation time for lunch is 3:30 pm. So, as long you have a reservation for at or before 3:30 pm, you'll be able to order lunch. Basically, it means they stop seating new guests for about an hour (between 3:30 pm and 4:30 pm) allowing the kitchen to finish serving lunch guests and start prepping for dinner.