Ni-chome is the biggest gay town in Japan and it only has own promotion association. They hold street festival named “Tokyo Rainbow Festival” in bon season every year since 2000.
And this  “Ni-chome Yukata Festival” is the realational event of Tokyo Rainbow Festival. If you wear Yukata or Jinbei and go bars which is member of Ni-chome promotion association, you can get some benefits like another drink or original present.
So you should join this event wearing Yukata tonight.

※Business hours and benefits depend on each bar. The map of member bars is here.