You can schedule Apex classes to run at specific times. First implement the Schedulable interface for the class, then specify the schedule using either the Schedule Apex page in the user interface, or the System.schedule method. The Schedulable interface contains one method that must be implemented, execute. Here's an example:

global class scheduledMerge implements Schedulable{ 
  global void execute(SchedulableContext SC) {
     // Do some work
  }
}

The following example uses the System.Schedule method to implement the above class.

ScheduledMerge m = new ScheduledMerge();
String sch = '20 30 8 10 2 ?';
system.schedule('Merge Job', sch, m);