one's best clothes; one's only good outfit; Sunday best
Refers to the one special outfit a person owns for important occasions, often implying it is the only presentable set of clothes. Can be used humorously or self-deprecatingly.
I wore my best suit to the class reunion.
彼の一張羅は、就職活動のときに買ったリクルートスーツだけだ。
His only good clothes are the recruit suit he bought for job hunting.