Is Saturday considered as a Rescission Day when my bank in Texas is not fully operational on that day?

Yes. See Reg. Z 226.2(a)(6).

Reg Z - Section 226.2(a)(6) "Business Day" means a day on which the creditor's offices are open to thepublic for carrying on substantially all of its business functions. However, for purposes of rescission under sections 226.15 and 226.23, and for purposes of section 226.31, the term means all calendar days except Sundays and the legal public holidays.

The way I remember it is any day that mail is delivered is a business day for purposes of rescission.

That is a rule of thumb but is not always accurate.

July 4th falls on a Sunday, the post office and your financial institution closes on Monday July 5th to observe the holiday. For the ROR Monday July 5th is still a rescission day even though there is no mail delivered and you are closed.

http://www.bankersonline.com/tools/compliance/rescissioncal.html

Dan, Wouldn't Monday then be considered a legal public holiday and therefore not be a rescission day?

No because the regulation specifically states '4th of July' and not Independence Day.

No. See 5 U.S.C. 6103(a), the holidays that are designated by a specific date are holidays only on that specific date for the purposes of rescission.

http://www4.law.cornell.edu/uscode/5/6103.html

Well, you learn something new everyday! Thanks.