Outlook.com and Office 365's Exchange Online accounts have separate Birthday and Holiday calendars. You can remove the calendars if you aren't using them but you cannot delete individual items from the calendars.
I am trying to delete a list of birthdays, but it won't allow me. A message pops up "You cannot make changes to content of this read-only folder"
The server created the birthday calendar from your contacts and it is read-only; you can turn it off in Outlook on the web's Calendar options but cannot delete individual birthdays from the calendar. If you want to remove a birthday from the calendar, you'll need to edit the person's contact.
To remove the Holiday calendar, right click on the calendar and choose Delete folder. You can delete the Holiday calendar in either Outlook desktop or Outlook on the web.
These calendars sync with your mailbox. When you delete them from Outlook on the web, they will be deleted from all devices your mailbox syncs with.
Enable the Birthday or Holiday Calendar
To enable (or re-enable) either Calendar, you'll need to log into your Outlook on the web account then: