Calendar days is the preferred choice. Everyone knows what that means. Business days under REBBA only apply to regulatory matters related to registrants. It does not apply to Vendors and Purchasers. So, iit is a very problematic term. Banking days is just as bad. There is no definition of that term either. Best advice use precise dates and times, if you have to use a number of days, then use calendar days. If you are using either business days or banking days, then include a definition in your APS.