I based this script off of one referenced here : http://blogs.msdn.com/b/emeamsgdev/archive/2012/10/29/outlook-code-importing-bank-holidays-from-an-outlook-hol-based-csv-file.aspx?CommentPosted=true#commentmessage

CSV Format

However I modified Adding a Category to the item, as well as adding this to All Users of a AD Group recursively instead of a single user. This will add a list of Holidays to Users Calendars across an organisation. If run again it will not import duplicates and have fields there to delete the duplicate Calendar enteries if needed

The user running this will need the impersonation role setup see this :

New users can then have an email sent to them automatically ( http://blogs.technet.com/b/sukum/archive/2013/04/03/send-welcome-email-to-new-mailbox-users.aspx ) with a .HOL file to run ( P.S. The XML did not work in this , I have to add Lines 6-8 from this http://itnuts.wordpress.com/2013/09/03/exchange-2010-automatic-welcome-message-for-a-new-user/)

https://github.com/pariswells/pariswells.com/blob/master/saveappointmentintoalluserscalendar.ps1