Reason to use:
- Capture a dynamic sequence of operations needed to carry
  out a task.
- Want to create a sequence of comands that are executed
  in order. The executor does not need to know anything
  about what the commands do or how they do it. The Executor
  simply tells each command, in order, to execute.
- Supports Undo/Redo functionality.

http://www.oodesign.com/command-pattern.html

Calendar events
Schedule for account operations (ie transfer from/to on this date)
Enables Undo

Alternate: https://dzone.com/articles/design-patterns-command
