The Day Before Yesterday is a short memoir of her year at Prince of Wales College in Charlottetown that L.M. Montgomery published in the May 1927 issue of The College Times. It is reprinted in The L.M. Montgomery Reader, Volume 1: A Life in Print.
Bibliography
MacDonald [sic], L.M. Montgomery. “The Day Before Yesterday.” The College Times (Charlottetown), May 1927, 29–34.
Also in The L.M. Montgomery Reader, Volume 1: A Life in Print, 230–36.