needphphelp

Hello I am trying to help a small company out by creating a simple dispatching script. Here is what im stuck on.  I need a way to have the customers be able to pick a day from a calendar and have that day be placed in the appointment table with the rest of their appointment information.  The calendar just needs to be very, very, very basic. creating the layout for the calendar is not the problem, what is the problem is only the days with available appointments should be able to be clicked. Say each day has 12 available appointments - those days which have 9 or less would be green and clickable, those with 10 and 11 would be yellow and still be clickable, those greater than 12 would be red and unclickable. All that needs to happen when the day is clicked is it needs to add +1 for the total count and then write the date that is clicked into a session variable, which will be added to the database with the rest of their appointment information. I am not really looking for someone to come up with all the coding - just for some design tips.  For some reason I am having trouble even brainstorming of how the whole process would be laid out.  I have never worked with the calendar function, but it should be interesting learning :) Any help on the subject would be greatly appreciated and a big THANKS in advance to everyone!  once again, i am not simply looking for a compelete code, just another mind to brainstorm of how this could work.