For me, "Sunday best" means neither work nor play clothes, but clothes you don't want to get dirty, not formal like black tie, but the best you'll look all week.

Then again, we have a beach on Kauai where on Sundays many beach goers are in their "Birthday Suits" but I'm thinking you don't mean that, haha!

When I was a boy (I am 64) a person who looked good was decribed as "Cadillacking." Or, "You be stylin'."

It means nice clothing. I see no reason to get rid of it any more than getting rid of Thursday.

The general idea in Christianity is to show respect for god by reserving your best clothing / appearance / conduct for worship. Or something along those lines.

When I was growing up in the evangelical world back in the 1970s, it didn't mean much except to the oldsters even then. Might be different in the mainline denominations or more affluent congregations, I don't really know. I certainly wore my nicer stuff to church but as a teen at least never did the tie and suit thing.

I think it works as a turn of phrase. It "communicates", which is what matters. We can admit, as atheists/agnostics, that we were/are raised with judeo/christian values and social norms. It makes sense to have colloquialisms that are appropriate to our histories. It's romantic. The phrases will change in time. In the recent social justice movement, it is tempting to want to wash history away. If you Just stop using the term yourself...and you are living as Gandhi suggested, "be the change," then that works very well. Others will use it less if you stop. Otherwise...make up your own phrase... it'll "communicate" too. like "dress as nice as you can" or "as close to black tie as you got" Or... "wear clothing that has been shown to reduce bloodflow to the brain through constriction." ?

Shiny shoes event suit.

I have a car that I referred to as my sunday-go-to-meeting car never said where I was going to meet

I remember 'Sunday drivers', people in NJ/NY who only drove around on weekends, enjoying the sights, driving other drivers slowly nuts.

Sunday best.... My pajamas and no where to go.