in (one's) glad rags
Dressed in one's fanciest clothes. This isn't a formal event, so there's no need to be in your glad rags. A group of teenagers in their glad rags boarded the bus on their way to the prom.
in your glad rags
in your smartest clothes; in formal evening dress. informal 1922 H. B. Hermon-Hodge Up Against It In Nigeria We all turned out in our glad rags to join in the procession.