Question: I’ve heard conflicting things about whether we should wear Shabbos clothes on Shabbos Chazon. What should we do?
Answer: The Shulchan Aruch (OC 551:1) writes that we should limit our rejoicing from Rosh Chodesh Av. Thus, the Rema writes that one shouldn’t wear Shabbos clothes on Shabbos Chazon (Shabbos before Tisha B’Av).
The Mishna Berura (551:6) writes, however, that the Vilna Gaon and Yaavetz both held that one should wear their Shabbos clothes as normal (See Chayei Adam 333:1; Kaf Hachaim OC 551:13).
The Aruch Hashulchan (OC 551:11) explains that people wear Shabbos clothes as one isn’t allowed to publicly display acts of mourning on Shabbos (See Shulchan Aruch YD 400:1). He explains that the Rema was writing about a time and place (Cracow, 16th Century) when one’s Shabbos clothes typically looked no different to one’s weekday clothes even if they were of better quality. In 19th Century Europe, however, where people dressed differently on Shabbos than they did during the week, one who didn’t wear their Shabbos clothes would be publicly displaying mourning.
He writes, however, that where one’s Shabbos and weekday clothes are similar, one should wear one’s weekday clothes.
Especially as people are generally more fashion conscious today, one who wore their weekday clothing on Shabbos Chazon would probably be noticed. It is possible, therefore, that even the Rema would agree that nowadays one should wear their Shabbos clothes.
In conclusion, one should wear one’s regular Shabbos clothes on Shabbos as doing otherwise would be showing a public sign of mourning on Shabbos.