Oh come on, use a bit of imagination here. It could have been some code for someone to see... "on 9/" might be very specific instructions for a hit, for instance. Or perhaps 9/ is the name of a person that lives in his/her head, and they felt compelled to cheer them on. on 9/! and you go ^3! Who da man?
on 9/29, i get very stoned and bored and stumbled onto wasteland express.