This fifth in a series of reports concerning school calendars examines the literature about learning, retention, and forgetting. The emphasis is necessarily on "time," as that is the major variable among calendars. Theories of memory are described: (1) storage and retrieval, and (2) imprint and trace decay. Relative to forgetting, a secondary concomitant is interference. The functioning of interference with retention and forgetting is explained. The report points up differences in the learning rates of advantaged and disadvantaged children. It also shows the summer losses that disadvantaged children have when the schools are closed for the summer. The appendix summarizes representative forgetting and retention studies. (Author/MLF)