Hi Lindsay,
From my understanding, there won't be a specific "Calendar - Read Only" permission (though I wish there was, it would be so much easier!). You will need to have the Calendar permission enabled in order to access the Calendar tile on the My Marketo page in order to view it.
However, if you want a specific user to be "Read Only" on the calendar, you would enable the Calendar permission as part of your five free users and make grant access to view other sections of Marketo (Marketing Activities, Design Studio, etc), but not Create/Update/Delete.