Access the Google calendar in your browser, click the + icon next to 'Other calendars' to add a calendar, 'Browse calendars of interest', Regional holidays, Browse all, select Holidays in United States. In TB, add a new calendar 'On the network', enter your gmail address..., then select the holiday calendar.

It was easy to subscribe to Google's holiday calendar with @fshowes instructions. In case it's not obvious to everyone, here is a more detailed how-to:

• New Calendar (lower left)
• On the Network
• Enter Gmail address as "Username," "Location" automatically fills in as gmail.com
• Sign-in process (gmail + password + verification)
• "Create New Calendar" dialog shows available calendars
• Select the calendar(s) you want.

I found it easy to follow the instruction from sfhowes, but in case anyone would like a more detailed how-to:

• New Calendar (lower left)
• On the Network
• Enter Gmail address as "Username," "Location" automatically fills in as gmail.com
• Sign-in process (gmail + password + verification)
• "Create New Calendar" dialog displays available calendars
• Select the calendar(s) you want.