How do we make calendars? This lecture explores the astronomical origins of our calendars. We start by discussing lunar and solar calendars and their hybrids in history and tradition (for example, the Islamic Lunar Calendar and the Hebrew Luni-Solar Calendar), and then describe the Julian and Gregorian Calendar reforms that attempt to align the calendar with the seasons of the year with greater degrees of precision. Recorded 2007 Oct 3 in 1000 McPherson Lab on the Columbus campus of The Ohio State University.