when does elena turn her humanity on

Elena turns her humanity back on in The Vampire Diaries at the end of season 4 when Damon kills Matt Donovan in front of her. This event triggers Elena's grief and emotional response, which reactivates her humanity after she had turned it off to cope with the overwhelming pain of losing her brother Jeremy. The plan to turn her humanity back on was created by Matt, with Damon's assistance, and it works by piercing Elena's heart with powerful emotional pain, specifically the shock of Matt's apparent death. Prior to this, Elena had turned off her humanity to avoid feeling the intense grief and became ruthless and emotionless, but after seeing Matt hurt, her humanity switch was flipped back on.