How do we manage Important Milestones (or Events) across all the Projects (or visible in every collaborator calendar)

How do we manage Important Milestones (or Events) across all the Projects (or visible in every collaborator calendar)

There should be one calendar visible by all collaborators. 
If one team member wants to add a milestone that should be known by all the team members, (for instance to make sure that they don't plan important delivery the same day). How does that member proceed ?