Chapter 7 Firewall for FortiOS 5.0 : Firewall objects : Firewall schedules : Schedule Groups
  
Schedule Groups
You can organize multiple firewall schedules into a schedule group to simplify your security policy list. The schedule parameter in the policy configuration does not allow for the entering of multiple schedules into a single policy so if you have a combination of time frames that you want to schedule the policy for then the best approach, rather than making multiple policies is to use a schedule group.
Example
Your Internet policy allows employees to visit Social Media sites from company computers but not during what is considered working hours. The offices are open a few hours before working hours and the doors are not locked until a few hours after official closing so work hours are from 9 to 5 with a lunch break from Noon to 1:00 p.m.
Your approach is to block the traffic between 9 and noon and between 1:00 p.m. and 5:00 p.m. This means you will need two schedules for a single policy and the schedule group handles this for you.
Schedule groups can contain both recurring and one-time schedules. Schedule groups cannot contain other schedule groups.