When creating a new calendar, by default Simple Calendar can get events from a calendar in Google Calendar app. You can add any calendar that you own or have subscribed to, as long as this calendar is public. Calendars in Google Calendar application can be either public or private. The simple API key you will be using to fetch events from a Google Calendar only allows to read from public calendars. Adding a Google calendar which is not public won’t display events in your calendar added to your site.
By default, calendars in Google Calendar app are created private, except those of public utility you (such as national public holidays, sports events) you may have subscribed to. Third party calendars may or may not be public – but if they don’t belong to you, you cannot change their privacy setting.
You can make any calendar that you own public by following these instructions.
1 Open your Google Calendar app page using your Google account.
2 Navigate to your subscribed/available Google calendars list (usually bottom left side).
3 Click on the dropdown icon on the right of a calendar title. A menu will appear. Choose and click Calendar Settings.
4 A new page will open. Click on the Share this calendar tab.
5 Tick the option Make this calendar public.
Now your calendar is public and can be added to a calendar.
Please note that by doing so your calendar might be reached also through Google or by whomever has the calendar ID or a link to your calendar on Google.