It is customary to have a a meal before Tisha B’Av begins, known as the Seudas HaMafsekes. One shout ideally sit on the floor for this meal, or at the very least on a low stool, or pillow. One should only eat plain bread and ideally only drink water. If need be, a person may have a coffee or tea at this meal. The meal should be not be eaten in a communal fashion but rather each person should sit by themselves. It is the custom to eat an egg dipped in ashes at this meal. All of the customs of the Seudas HaMafsekesare are meant to reflect a sense of mourning.
This does not mean that one cannot have a large meal earlier. If one wants to eat a large meal they could certainly do so. However, they should eat that large meal with enough time to wait a half hour after the large meal before beginning the Seudas HaMafsekes.