"Sunday best" is a dress code suggestion that's dressier than casual wear but less formal than black tie. Similar to what you would wear to a church service. It is just put on the invitation as a suggestion of not needing to wear your formal attire to the ceremony, but you are welcome to!
“Black Tie Optional” gives guests the choice of wearing traditional black tie attire (a tuxedo for men, an evening gown for women) or opting for a formal suit and a floor-length or fancy cocktail dress. Dress to impress!
We kindly request our guests to wear all black attire to the reception. As it is NYE, we encourage glitz and glamour!