Sunday best

idiomatic uncountable

(idiomatic) A person's finest clothing, especially the clothes one reserves to wear to church on Sunday.

Japanese Meaning

（教会に行く日など、特別な日に着る）最高級の、または最も上質な服装 / 一番良い・格式高い服装、特別な席で着るための最も良い服

She put on her Sunday best before walking into the old church for the wedding.

彼女は結婚式のために古い教会に入る前に晴れ着を着た。

Sense（1）

(idiomatic) A person's finest clothing, especially the clothes one reserves to wear to church on Sunday.

And how then was the Devil drest? / Oh! he was in his Sunday's best