February 6, 1954: You know the thing I like best in the world? CAUSTIC SODA

Read this strip at gocomics.com.

There is spite, and there is this.  There's something almost affectionate about holding an entire party specifically to focus on something a particular, pointedly-uninvited person likes, although it's rather a lot of trouble to go through.  I can't help but think that the result would be rather a slapdash affair.  Why would you do otherwise, when the "guest of honor" won't even be there?

April 29, 1952: The birth of Charlie Brown's insecurity

We've seen hints of it before, but this is the first one in which Charlie Brown seems actually delighted just to be included with the others', like by default he is some kind of pariah, shunned by all.

This is also the first strip in which Lucy is used as a background character. It might also be the first strip in which Schroeder is used as such. That's an important step towards promoting them to full kid-hood.