This episode coupled with the next episode, "Dreams Come True" originally aired the same night (March 20, 2015) and together they make up the series finale.
The song Lea Michele and Chris Coffer sings as a duet, "Popular" from the Broadway musical "Wicked", was originally sung by Kristin Chenoweth who plays the recurring character April Rhodes on "Glee".
Rachel speculates that Kurt is a contralto or mezzo-soprano. However, this would be an informal designation; those voice types are classically defined as female, and Kurt would be called a countertenor, the highest male voice type.
You can tell that Kurt and Artie's "flashback" scenes (especially theirs) were filmed in 2015 because of how their hair (and the shape of Chris Colfer's face) are different from the scenes actually filmed in 2009.
This episode takes place in 2009.