Google Calendars has public calendars which show public holidays. You can use this calendar along with your own calendar.
Add Holidays Calendar
On your computer, open Google Calendar.
In the left menu, click plus icon next to the "Add a friend's calendar" input.
Choose "Browse calendars of interest"
Select the appropriate calendar you can add. It is then added next to your own calendars on a calendar menu on the left.
Click the name of the calendar you want to use.
In the "Integrate calendar" section of the screen, you will see your Calendar ID. It will look something like "firstname.lastname@example.org".
Paste the link to the dashboard gadget configuration page.