Both Office 365 and Outlook.com users have a separate Birthday calendar in their mailbox. The server populates the calendar from your contacts, in much the same way that Outlook creates birthday and anniversaries for your contacts, but without adding more noise to your own calendar. Because it's a separate calendar, you can hide or show the events easily.
However, a lot of users don't like the extra calendar, especially when they already have the birthdates on their default calendar, thanks to years of using Outlook. Fortunately, it can be deleted from Outlook, which turns it off in Outlook on the web.
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.
If you decide to keep the separate birthday calendar, you'll probably want to delete the birthdays from your own calendar. The easiest way is to search for "birthday" or "'s birthday" then Select All and Delete. Search for "location:country name" if you need to delete Holidays.
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
If you want to restore either Calendar, you'll need to log into Outlook on the web if you want to enable it again. You can enable it from the Add calendar link in the bar or in Calendar Options, Other Calendars.