The strip from February 26, 1967.
Cold cereal is a running gag in the Peanuts comic strip. The gag would show any character like Charlie Brown or Linus doing something like calling on the phone or studying a book, the end result in the final panel is the characters saying "Cold cereal!" or "My cold cereal is getting soggy!"
Another cold cereal reference is in the strip from August 1, 1987 where Marcie kicks Charlie Brown, The cereal is still cold at his home.
This gag would continue to appear until December 24, 1999, when Charlie Brown suggests to his sister Sally that they should make Christmas cold cereal.