Q. Is it better to eat meat on this shaleshudes (when Tisha Beav is Sunday) when you don’t do so the whole year?

A. Shulchan Aruch (O.H. 552: 10) rules that when Tisha Beav is a nidche one can eat on all the Shabbos meals meat and drink wine. He can make his Shabbos seudos as extraordinary as the banquets of Shlomo Hamelech in his day. However, Poskim do not mention that one is obligated to do so. On the contrary, Nitey Gavriel (H. Bein Hametzorim 2: 91: 4) quotes some Poskim that maintain that one should eat dairy (Zera Kodesh et. al.)
Horav Shlomo Miller’s Shlit”a opinion is that one should eat what is pleasant, with the intention of complying with the mitzvo of Oneg Shabbos. Although one eats also with the aim of preparing for the fast, he should not mention this openly, as it would constitute an act of hachono or proscribed preparation from Shabbos to weekdays.

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit”a