If like me you have found that your Apple supplied public calendar subscriptions (such as “US Holidays”) has stopped refreshing, you may wish to try my fix: right/context click on the calendar, choose “Get Info” and change:
http://ical.mac.com/ical/US32Holidays.ics
to:
webcal://ical.mac.com/ical/US32Holidays.ics
That worked for me. Or you can just mosey on over to http://icalshare.com/ and subscribe to the calendar(s) of interest from there.