2004 article from Education Week on Year-Round Schooling.

This is an overview on how time is used in schools.

This is an article from the Center for Public Education (cpe) summarizing research on time variations in schools around the world.  It also looks at the history behind school calendars. 

National Education Association reviews of the research on Best Practices in Education.  This review focuses on year-round educational practices.

Summary of research on year-round school schedules.