A milestone, by definition, has zero duration and no effort. A milestone is a marker in your schedule. You don't place milestones in your schedule based on a calendar event. In other words, you don't schedule a milestone for the first Friday of every month.
Milestones are great for managers and the sponsor because they provide an opportunity to validate the current state of the project against the overall schedule. Since each milestone signifies that some set of underlying work has been completed, your sponsor should know immediately that your project is behind schedule if a milestone date is missed. The sponsor does not need to know the individual status of all the activities in the workplan. He just needs to keep track of the status of the milestones to know if a project is on schedule or not.
In addition to signifying the status of the project against the workplan, milestones also provide a great way to take a step back and validate the overall health of the project. In particular, the following types of activities can be scheduled for (or at) each major milestone.
These activities should be done on a regular basis, but a milestone date is a good time to catch up, validate where you are, get clear on what's next, and get prepared to charge ahead.