1,390. Eating Before Tisha b'Av

Hilchos Taaniyos 5:7

All the Tisha b’Av restrictions apply on the preceding night just as they do by day. One may not eat starting from sunset. It is prohibited even between sunset and when the stars appear, just as it is on Yom Kippur. One may not eat meat or drink wine at the meal preceding the fast but one may drink grape juice that has not been left sitting for three days and one may eat salted meat that was slaughtered more than three days before. One may not eat two cooked dishes.

Hilchos Taaniyos 5:8

The above halacha applies when one eats in the afternoon on the day before Tisha b’Av. If one eats before noon, he may eat whatever he likes even if it is his last meal before the fast commences. When the day before Tisha b’Av is a Shabbos, one may eat and drink as much as he requires, even serving a meal as lavish as one of King Solomon’s banquets. Similarly, if Tisha b’Av falls on Shabbos, one need not restrain himself at all.

Please note that the practices described by the Rambam may not reflect our current observances.