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Do you share any calendars with anyone? Or did someone perhaps invite you to an event? Open Calendar and check for an icon with an arrow pointing down into what looks like a box in the upper left hand corner of the window. If you see that with a “1” or other number on it, that’s where the badge on the calendar icon in your dock is coming from.

Otherwise, it’s possible for badges to get “stuck” from time to time but a simple restart of the Dock with terminal will fix that.

Open Terminal (search Spotlight for it) and type in “killall Dock” and press enter. The dock will disappear and then relaunch, and the badge should be gone.
 
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