Subscribe to public calendars using the calendar address
To add a calendar using its address, follow these steps:
Click the down-arrow next to Other calendars.
Select Add by URL from the menu.
Find the address of the Google Calendar in iCalendar format, or if you have the iCalendar address of a calendar from a different application (for example, Apple's iCal), you can use this as well.
Enter the address in the field provided.
Click Add calendar. The calendar will appear in the Other calendars section of the calendar list to the left.
Note: It may take up to 8 hours for changes in ICS feeds to reflect in your Google Calendar.