Google Calendar has a number of generic calendars that you can add that give all public holidays for a specific country. After adding this, you can display it along with your own calendars to see when holidays are approaching.
To add a holiday calendar:
Click on the + symbol to the right of the Other calendars heading on the left-hand side of the screen
Choose to Browse calendars of interest
Click on the Regional Holidays link
Check the box to the right of the country you wish to add
Click on the Back button to return to the calendarÂ