What episode and season was glee singing summer night?

Glee did not sing "Summer Night" from "Grease". They did sing other songs from the musical, including:

* "Grease" (Season 1, Episode 16, "Sectionals"): This was a mashup with "You're The One That I Want."

* "Hopelessly Devoted To You" (Season 1, Episode 17, "Regionals"): This was part of a medley with "We Go Together."

* "You're The One That I Want" (Season 1, Episode 17, "Regionals"): This was part of a medley with "Hopelessly Devoted To You."

"Summer Night" wasn't part of any episodes of Glee.