Having played around with the parameters added from the embed tool in Google Calendar, I figured out how to add holidays, for example, to the URL that links to a shared calendar.
This URL uses Christian and U.S. holidays:
http://www.google.com/calendar/hosted/[domain]/embed?src=admin%40pumyf.org&color=%23A32929&src=en.christian%23holiday%40group.v.calendar.google.com&color=%23AB8B00&src=en.usa%23holiday%40group.v.calendar.google.com&color=%232952A3&ctz=America%2FChicago
