12 CFR § 229.15(b) addresses the uniform reference to day of availability. The commentary specifically says...
Banks must disclose when deposited funds are available for withdrawal by stating the business day on which the customer may begin to withdraw funds. The business day funds will be available must be disclosed as “the ________ business day after” the day of deposit, or substantially similar language.
The regulation is pretty specific, so I'd use their exact wording. However, I do encourage my employees to note the actual date off to the side so that the customer doesn't have to guess what day they can withdraw the funds. I point out to my employees that if they struggle to count the days, surely the customer will, so we do it occasionally as a customer service tool.
Don't make me say, "I told you so!" Sincerely, your friendly Compliance Officer.