Can any one clarify for me please...in reading Reg Z’s rules on Rescission, do I understand it right that a business day is defined as any day that we are open for full business except Sundays and Holidays unless we are calculating for Rescission we have to include Saturdays?

Word Up, for rescission purposed you DO include Saturdays

When you're talking about ROR, whether or not you are open is irrelevant. Notice the bolded part of the definition of "business day" (Sec. 226.2(a)(6)):
(6) Business day means a day on which a creditor's offices are open to the public for carrying on substantially all of its business functions. However, for purposes of rescission under §§ 226.15 and 226.23, and for purposes of § 226.31, the term means all calendar days except Sundays and the legal public holidays specified in 5 U.S.C. 6103(a), such as New Year's Day, the birthday of Martin Luther King, Jr., Washington's Birthday, Memorial Day, Independence Day, Labor Day, Columbus Day, Veterans Day, Thanksgiving Day, and Christmas Day.