Many of the timing requirements found in Regulation E are based on “business days”. There are some places in the regulation, however, where only “days” are mentioned. Thus, it’s very important that institutions understand the difference. When the regulation mentions days, this generally means calendar days. Business days, on the other hand, are defined in §1005.2(d) to be …any day on which the offices of the consumer's financial institution are open to the public for carrying on substantially all business functions. Based on this definition, business days could differ from institution to institution. You are required to disclose your business days in your initial Electronic Funds Transfer disclosure.
David explains more in the video.