This year, Tisha B'av falls out on Shabbat. The Gemara states that in such a case the fast is pushed off to Sunday, the tenth of Av. The fast is postponed and not moved forward in accordance with the principle of not hastening tragic event, such as this fast marking the destruction of the Bet Hamikdash. In a regular year, the last meal before the fast is known as the "Seudat Hamafseket," and it is limited to a certain number and types of foods. However, this year, the last meal is a regular Shabbat Seudah Shelishit. One may even eat a meal fit for Shlomo HaMelech, as the Shulhan Aruch states.
From sunset, a person is in a state of "limbo." On one hand, it is already prohibited to continue eating and drinking. On the other hand, it is still Shabbat, and therefore one must not make any public display of mourning or preparation for the fast. For example, one should not put on his non-leather Tisha B'av shoes while it is still Shabbat. Rather he must wait 40 minutes after sunset, say "Baruch Hamavdil Ben Kodesh L'Hol," and then he may change his shoes and come to the Bet Knesset.
The Halacha prohibits "Hachana," preparing from Shabbat to the next day. The Poskim discuss whether one is allowed to drink excess amounts of fluids throughout Shabbat in order to be properly hydrated for the fast. Is this considered Hachana? They rule that one may do so, as long as he does not verbally express that his intent that it is for the fast. The same applies for eating in preparation for the fast.
Since Tisha B'av is on Sunday this year, the usual restrictions of "Shabua Shehal Bo" (the week of the fast) do not apply, as there are no days preceding the fast. Therefore, it is permitted to wear freshly laundered clothes the week before the fast. However, on Tisha B'av itself, it is certainly prohibited to wear freshly laundered clothes. Accordingly, one must be careful that when he changes out of his Shabbat clothes on Saturday night that he dons weekday clothes that have already been worn.
