Abstract: In a scheduling system, events and/or groups of events are checked at a scheduling time to insure that certain fixed conditions associated with the event(s) are satisfied. The events are also checked at one or more times, between scheduling time and a performance time (when the event(s) are to be performed), called "prepare to perform time(s)." At the prepare to perform time(s), certain dynamic conditions and/or data associated with the events are checked to determine whether the dynamic conditions are satisfied. If the dynamic conditions are satisfied, the event(s) are confirmed for performance. If one or more of the dynamic conditions are not satisfied, the event(s) are modified. Events can be modified by cancelling, altering or postponing. When an event(s) is modified, a notification can be sent out. Further, a modification of a event(s) can cause modifications to one or more subsequent events in the event group (propagation.