From this page:
Contacts' birthdays and events
When you subscribe to this calendar, all of your contacts' birthdays will display as All Day events in Google Calendar. Data is pulled from Google Contacts, which you can access via http://www.google.com/contacts (or, if you have a Gmail account, click Contacts from your inbox).
Have you tried removing the date from your Gmail contacts?
If you go to your "System Preferences" -> "Internet Accounts" -> Google or Facebook as examples. There will be check or uncheck boxes for Contacts and Calendar. If they are checked this will any event to your iCalendar. You need to uncheck them.
Or you can totally disable the account at the lower bottom left corner "Enable this Account" and uncheck it.