I'll have to check the actual keyboard to be sure, but I at least have documented that I have a keyboard from May 24, 1992, which was a Sunday. It's possible that I copied the date wrong, but I can take a look later today.
And, thinking about it, I was born on a Sunday and I'm pretty sure I saw a keyboard with that day. Exactly once. It was an SSK, and I wasn't interested in/willing to spend for one. So I'm guessing they're rare, but that some may exist for at least some Sundays.
(Now I'm wondering if it's normal to have a spreadsheet documenting the manufacture dates of keyboards you own...)