The current 9-month school calendar emerged when 85 percent of Americans were involved in agriculture. Today, only about 3 percent of Americans' livelihoods are tied to the agricultural cycle, and air-conditioning makes it possible for schools to provide comfortable learning environments year-round. Noting the National Education Commission on Time and Learning report, which reflected a growing concern about the impact of a long summer break on at-risk students, this Digest examines loss of learning over summer and offers suggestions for addressing the problem. The digest first examines concerns raised by long summer vacations: children learn best when instruction is continuous; the long break affects special needs students, such as those learning English as a second language or those with disabilities; and equity concerns, as higher SES students often return from summer break with considerable educational advantage. The Digest next summarizes research into summer learning loss. Meta-analyses are cited, which found that summer learning loss equaled at least one month of instruction as measured by grade level equivalents on standardized test scores, that summer loss was more pronounced for math facts and spelling--both factual/procedural rather than conceptual learning; and that individual differences among students may also play a role. Finally, the Digest details three approaches to preventing summer learning loss: extended school year, summer school, and a modified calendar that replaces the long summer break with shorter cycles of attendance and breaks. (Contains 15 references.) (HTH)