Unwanted NYE notification on iPhone

Starting the middle of last night, New Years Eve, and continuing today, I've been hearing what sounds like a notification, although it doesn't match the sound for new text messages. Is there a built-in NYE notice? That'd mean it will go away by itself, which would be nice. It is not accompanied by an on-screen text of any kind, making it doubly mysterious.

[Re-Titled by Moderator]