Sunday best

Yes, it is still frequently heard, although it is often used slightly ironically or tongue-in-cheek (i.e. given that literally dressing up for church on a Sunday is increasingly outdated).

Still used in the non-ironic sense here in Texas, if the speaker wants to give a sort of old-fashioned, earnest tone to what he's saying.

Thank you Dave and Speedwell for your usual prompt help!

Well, in Iowa, I usually never hear it. When I do, it's mainly said by elderly people on Sunday after they've been to church.

Yes, it is, but the difference is that many people in my generation (I'm 22) or younger may consider one's "Sunday best" to be a lot different (i.e. more casual, less conservative, etc.) than what was once considered to be such.

I personally use my own phrase "recital clothes" to connote what once might have been understood as "church clothes." It's a more prudent style implied. "Nice clothes", however, are frequently regarded as "work clothes" in the white-collar understanding.

More than you asked, sorry :).

No Sarah, that's what I asked! :)

I really appreciate the perspective you threw on this for me.

Thank you

Edaurdo

this phrase is really only used if you are and older person or a christian, otherwise some people might think you are weird.

Always happy to toss in my two-cents. :) ~s

Someone I know is having an evening function and wrote to wear "Sunday best" on the invite. This is very confusing to me. What would you wear to something like this? Would a strapless A-line be appropriate?

You can use that word to compliment someone that looks awesome with his/her clothes on any occation or insult someone who looks like a fashion criminal with his/her wardrobe :)

Example:

Wow...you look beautiful today on your sunday best! (praising)

Aren't you look pretty on your sunday best(sarcastic)