When him and Vegeta were training in The artificial gravity chamber.
He Turned Super Saiyan back in his time when he found Future Gohan Dead.
Trunks turns Super Saiyan in front of Vegeta in the episode, "Take Flight Videl".
In the episode "Take Flight Videl" near the end of the episode
The First Legendary Super Saiyan was the first Super Saiyan because a legendary super saiyan comes every thousand years. And the first super saiyan is bardock because he traveled back time.
Episode 207, "Take Flight Videl", while at 150x gravity and vegeta punches him in the face. Hahahaa
That would be a matter of opinion completely because future trunks has been fighting his whole life not for sport but for his life but on the other hand our time-line's trunks became a super saiyan years before future trunks.
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He got the form during the 7 years between the Cell Games and Buu saga. First time shown was majin vegeta vs. goku. First time its used clean (without majin or any other influences) vs. kid buu
episode 286
"Roar" by: Katy Perry
When he was in the Hyperbolic Time Chamber training to beat Cell. I think he became Super Saiyan 2 after he died to save the world from Cell. If Goku could turn Super Saiyan 2 he would have beaten Cell himself, instead of making ohan fight him.