It's the canonical summer holiday, in the U.S. at least, so let's look at a couple of Christmas strips!
A brief touching upon the theme of the commercialization of Christmas, which of course provided the dramatic thrust of the Peanuts Christmas Special.
Schulz is moving away from bland celebrations of a holiday and towards more sophisticated jokes about it. The usual "YAY ITS CHRISTMAS" panel here is undercut by Schroeder's displeasure at being obscured. Note: no Shermy, Lucy or Linus in the last panel.
It is worth noticing that Patty and Violet are already starting to become a bit rarer. Lucy has usurped their roles, a little, as the strip's girl character.