Status.live.com has been showing the status below for Calendar for quite a while now.

When is the Outlook.com Calendar going to be operating normally again?

Is the automated synchronisation from People to the Birthday Calendar effected by the maintenance?

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"Outlook.com is performing maintenance on the calendar. You might see error messages when creating/editing birthdays. The checkbox to get notifications in the Options page may also be disabled during this time. Sorry for the inconvenience."