Nickelodeon (ended 2008)
I missed Avatar 3-17 and i found this: http://www.avatarchapters.org/62/book-3/chapter-17.html
In the preview, it shows aang and katara kissing, when did this happen?
Please use this thread for future questions.
But to answer your question, they kissed right before the invasion in The Day of Black Sun Part 1 and again in the second intermission of the play in The Ember Island Players.