What`s the meaning of "Sunday`s Best attire"?

Question

The children`s attire to be wear during there xtmas party.

Answer 1

It just means wear some dressy clothes, something that people would wear to church. (Nowadays people wear jeans and everything to church, but don't wear this if it says Sunday best.) It would be a nice dress or skirt outfit for a female and a suit or slacks, dress shirt, and tie for a male.

Answer 2

One's best and often most formal clothing.

[From the practice among Christians of wearing one's best clothing to attend church on Sundays.]