The Universal Festival Calendar is compiled and checked from various web sources. The principal calendar books and websites used here are these:
The Interfaith Calendar is comprehensive, and has clear definitions of holy days of various traditions. See http://www.interfaithcalendar.org/2005.htm.
Calendars of Various Cultures and Religions is especially good in outlining calculation methods. See http://www.kelsung.com/calendar.htm.
Calendar Zone has the most complete list of sacred calendar sites, at http://www.calendarzone.com.
The Mystic's Wheel of the Year: A Multifaith Calendar Reflecting Eco-Egalitarian Spirituality (Page Two, Inc.) has grown immensely in scope and quality