Now open on Sundays?

Working on an email this week and this quetion came up. Which is correct? "Now open on Sundays" or "Now open on Sunday." Both seemed like they could be correct to me.

So I pulled an ask-the-audience card and posed the question to my facebook friends.

The consensus? Both are correct. "If it's not discussed in Strunk & White I'm either asking the wrong question or won't find anything anywhere." (quote thanks to Matt McConnell)