Set Default Calendar Event Access Per Role

To do this, follow the steps below:

1. Go to Setup > Users/Roles > Manage Roles
2. Edit the role 
3. Under Preferences sub-tab, add "Default {Event} access Setting for new {Events}
4. Set the value to Public.
5. Click Add.
6. Click Save

Note: changes will only affect the new events that will be created after the settings has been changed.