Having closed more than 500 short sales in my career, I would note that many of these responses are either addressing the TREC agreement or discussing bank addendums. In many cases, bank addendums commonly modify terms (including waivers as noted as well as Seller (bank's) ability to change the closing date etc) in the original contract and are returned for signature by the buyer before the original offer will be ratified by the buyer.
Now, distinct from a foreclosure sale with addendum(s) from the bank, a short sale typical involves a written approval letter from the bank that is issue either by an on staff negotiator or possibly via a third party negotiator hired by the bank. In many cases, this written letter specifies among other things, the specific date by which the approval requires closing to occur. This may or may not coincide with the original contract. Our experience is its far better to work within reasonable terms than push an issue such as closing per original date and execute an amendment to modify the closing date as needed. If the bank can not facilitate a 29th closing you risk losing the approval all together (as odd as it sounds, we've seen it happen).
My supposition is bank desires to avoid having to staff during period between Christmas and New Years. Good luck.