You have the option of adding availability (standard hours worked) and exceptions (blocks of time that override standard hours - i.e. vacation, etc.) for instructors. The Availability / Exceptions feature prevents the necessity of creating empty 'place holder' classes to indicate when an instructor is available by adding visual color-coding on the Day View Calendar. See Calendars / Schedules.
A user must have the Availability permission in the Staff section to see the Availability tab. See System Management: Manage Users for more information.