Unknown calendar available only when adding a New Event (Sierra) This has to do with Calendar.app on Sierra (10.12.6).

In the left sidebar, I can see 6 calendars under "On My Mac." I understand why these calendars are here and I need to keep all of them.

However, when I add a new event to the calendar by double clicking on a day's square, I can click on the box that chooses which calendar should receive the new event. In the popup menu, I see another calendar listed under "On My Mac" that is a (seemingly) random 10-digit number. 

I would like to know how to delete this mystery calendar.

I can add events to this calendar, but it never shows up in the lefthand sidebar. I've checked the accounts that pipe into my Calendar.app (Exchange, Google) and don't see this mystery calendar anywhere.



Thanks!
This is bit complicated but the only way I know it.

Open ~/Library/Calendars

Leave it open.

Open Calendar > Preferences and set your weird numbers calendar as default.

Now add New event to it.

Go back to the ~/Library/Calendars and look which files (by date/time has changed. That is the one to delete.

Do not forget to change the Default back to your choosing.