I couldn't let the day pass without posting this here.
On this day in 1972 the first Star Trek convention was held and I think we can pretty much trace the birth of cosplay to that day, at least in the USA. And if you consider "Trekies" as cosplayers.
The organizers called it "Star Trek Lives" as the original show has been canceled 3 years prior. They were hoping to get 500 fans but over 3000 showed up! Many in Trek costume ( not nearly as many as its become) but each successive trek con more and more showed up in costume.
SO: HAPPY BIRTHDAY COSPLAY :)