When a caller asked about a whole day rather than a particular time — “have you got anything tomorrow?”, “can I come on Sunday?” — and there was nothing free that day, your receptionist could tell them the day was fully booked. If the real reason was that you do not open that day, that was simply untrue: it described a closed business as a busy one.
It also cost you the better answer. A caller told you are shut on Sundays knows to try another day; a caller told you are fully booked may well ring a competitor rather than ask about Monday.
Your receptionist now works out which it is. A day you do not open is described as closed, a day that is genuinely full is described as full, and it offers the nearest day you can actually see them either way.
A date further ahead than your diary is open for is now described as exactly that, rather than as a closure. If somebody rings in August asking about December, they are told it is too far ahead to book yet — not that you are shut that day, which you probably are not.
Where it cannot tell the difference it now says only that nothing is available, rather than picking a reason. Saying less is better than saying something wrong about your business.