Julius Peter Christian Petersen died on this date in Copenhagen in 1910. 

To an outsider attending a graph theory conference, it must appear that we graph theorists have an unnatural reverence for a little symmetrical picture of 10 dots and 15 lines (see figure). No matter what the title of the talk, odds are that sooner or later the speaker will draw it, and heads in the audience will nod in apparent obeisance.

The truth is quite different. In reality, few of us go in search of the Petersen graph. It is more that it stalks us, hiding behind many an innocent lemma, waiting to spring out at us. Sometimes it is in a good mood, and it marches proudly at the head of a new sequence of graphs with some fascinating property. Just as often, alas, it sits across our budding proof as a sullen and stubborn obstacle.

-- Brendan McKay, review of a book on the Petersen graph