June 28, 1957 (Snoopy first walks "normally" on two legs, after achieving two-legged walking on November 17 and 23, 1955 by adapting his upright "dance step", which he had been using since October 19, 1952)
June 29, 1957 (Schroeder doesn't appear again until August 4, 1957. This strip references Dwight D. Eisenhower.)