kycrazies... Last I heard it was Monday nights. It does change from time to time though.
I have posted my question on two different boards and your question is the only reply I have gotten. It's funny. Ultimate is awesome and people rave about him when they come home but they either don't remember what nights they saw him at the Piano Bar or just just don't care to be helpful with the info.
You'll find Ultimate Chocolate playing the piano on Tuesday, Wednesday, Thursday and Saturday at 9PM. On the activities schedule (link provided) he performs at the times listed as "Blazing Piano". http://couples.com/swept-away/activities-schedule.pdf. The last time we were there the repeaters dinner was on Monday.