About Exchange calendars
If your calendars are administered through Microsoft Exchange Server 2007 with Service Pack 1 or later (for example, through your workplace), in Calendar you have a main Exchange calendar named Calendar.
You can add calendars to your Exchange calendar account, but you won’t be able to see updated invitations or attendee responses for events in those calendars. Also, events in the additional calendars won’t be visible to delegates (people who share your calendar account) and won’t appear when users check your availability. Full calendar features are available only in your main Exchange calendar.