Como dizer “Roupa de domingo, de festa, de sair” em inglês

Also:

Someone’s “best bib and tucker” UK – (not modern)

1 - She wore her best bib-and-tucker.
2 - On the afternoon of the party, bib-and-tuckered guests are queuing at three entrances more than an hour before entrance time.

Sunday best
threads (old slang)

This reminds me of the song "Handbags and Gladrags":

"So what becomes of you my love
When they have finally stripped you of
The handbags and the gladrags
That your poor old Grandad had to sweat to buy you"

You can wear a suit to your birthday party, but don't go in your birthday suit.

to be in one's birthday suit = to be nude

Judy Friedkin, the song 'Rock around the clock' says, "put your glad rags on and join me hon'..."

That song must be 50 years old! It is nothing we ever said nor that I ever heard. Of course ,I heard the song many times and probably never listened to the words. It was just a creative expression in a song, just like we would never say to put any kind of rags on unless kidding around. Poetic license! It is quite a happy expression "put your glad clothes on..." It creates a picture in your mind of having a blast at a party. I like it . However it is not used.