I'm getting many clients say that the available dates is not obvious enough in the calendar. it only goes dark grey and the other ones are light grey. it needs to be a really visible colour.

Yes got the same feedback from clients. "Why is this color not exactly the same as in the explantion?"
I know this is a bit gaga but the clients pay for our services and the least we must not do is to confuse them unneccesarily. ;-)