Of course they can, since the books belong to the consignors and auction sites are more than willing to bend to the wishes to the consignors just in order to get their books.
I even personally know of one consignor who had all of his books slabbed back at the end of 2015 and then simply left them all with Heritage to auction off over the following 3 years, based upon his auction schedule that he wanted. So yes, it's always left to the discretion of the consignors.
Do you know if this is true for *all* consignors, or only the big boys?
So, knowing the stigma attached to December/Christmas auctions (or other allegedly "bad" selling times), consignors willingly decide to sell then?