Days on which tours are available should be managed from google calendar.
Tours are given on fixed timeslots that should be managed from google.
For example; 10:00 - 12:00 // 12:00 - 14:00 // 16:00 - 18:00
Visitor of the website can select date on which they want the tour, select a timeslot that is still available and book it.
After booking the timeslot should be marked as booked so no other people can make the same reservation.
All should be written in php and well documented.