You can know when a tutor is available by looking at their calendar on their profile page. Tutors keep their calendar up-to-date to reflect their most current availabile days and times.
Dates in White are days that a tutor is available for tutoring. Clicking on any of those dates reveals the specific available time durations for that day. These are the time durations for which you can book lessons with the tutor. Simply select the time you want lessons to start and end.
Dates in Gray are days that the tutor is not available to offer tutoring. It could mean that the tutor has been fully booked for that day, or he/she does not wish to teach on those days. These dates cannot be selected.