The September-to-June school calendar has been a fixture of North America for almost a century. Its origins have usually been told as an unexamined tale attributed to features of nineteenth century rural society. We challenge this interpretation by suggesting that multiple pressures arising from increasing urbanization influenced its roots. We present information on the importance of the summer holiday in the development of compulsory schooling in several North American jurisdictions, with the main evidence from Ontario, the most populous province in Canada. We suggest, along with Gold (2002), that this development had wider applicability in several Northeastern and Midwestern American states. Beyond the issue of having an accurate story line, we examine why there has been such resistance in recent times to changing the school year. The school calendar may be another example of an enduring institutional form referred to by Tyack and Tobin as a "grammar of schooling" that resisted fundamental change in the twentieth century. Viewing the school calendar's ties with changes over time in the construction of other clocks of society may enable us to rethink the format of the contemporary school calendar. Finally, we consider the school calendar as part of a larger, ongoing discussion of what constitutes effectiveness of schools.