Hello @Guy95,
That's OK. Usually people select the checkin date first and then the checkout one. You seem to be doing it the other way around.
To see the 15th as available just select check in first, then check out.
All the dates up to the 15th are unavailable so no one can check out on the 15th. When you select check out, the earliset possible date will be the 16th. If you select the 16th, then it'll ask for the check in date and so it will show the 15th as being available for check in.
Steve.