What is the best design pattern for a Task System?

Question

 Case 1:

 I am trying to create a task system that will be allow users to create or pick a task. Once that task is issued, the correct notification will be sent to the user who needs to complete the task.

 

Case 2:

 Task is automatically generated within the workflow based on what user is logged into the system and after an action is made, the notification is sent to the end user who needs to complete the task.

 

For the notification system I am looking into the observer pattern to solve the problem but I am lost combining the task system.

 

Any help is greatly appreciated.