Isn't it a subscription to a moon phase calendar? Check in the calendar list (click on the 'Calendars' button in iCal) if you have a subscription. If yes, delete it by right-clicking on it and selecting 'Delete'.
Hope this helps and regards,
Searching for "moon" and deleting did not work for me. My "moon phases" data was stored in the "home" folder under iCloud (upper left area of calendar). I unchecked the "home calendar" and the moon phases went away. BTW, I'm using Mountain Lion.