When did the turtles first meet Usagi Yojimbo in the comics?
 

And how was that encounter?

Turtle Soup #1, 1987. The story is Turtle Soup and Rabbit Stew. Nice title, huh? ;P
Leo falls into some space time continuum anomaly and runs into Usagi's world. He meets up with some ronin and fights with them, at thesame time Usagi is fighting off a group of neko ninja. Then during the fight Usagi and Leo meet, start to run aat eachother in a an attack, and then Leo is inexplicably sent back to his own dimension. Stan Sakai drew the story. :)