"Back to December" was positively received by critics, who regarded it as one of the highlights of Speak Now. Critics have praised the song for its expressive lyrics, its indelible melody, and Swift's vocal improvement. The song enjoyed commercial success in United States, debuting at number six on the Billboard Hot 100, and at number seven in Canada. Internationally, the song received moderate success, peaking at number twenty-four in New Zealand, and number twenty-six in Australia.

The song's accompanying music video was directed Yoann Lemoine, which laments the aftermath of a break-up between Swift and her boyfriend during one snowy morning. The music video garners mixed to positive receptions from critics, who praise the spectacular visual sense in portraying the somber yet melancholic mood although in general, its inadequate narrative is being criticized. Following promotion, Swift has performed "Back to December" in several live appearances, notably at the 44th annual Country Music Association Awards and at the 38th American Music Awards.

According to Swift, "Back to December" is an apology to a former lover in the form of a song, something that she has never done before. She continued, "I've never felt the need to apologize in a song before. But in the last two years I've experienced a lot, [including] a lot of different kinds of learning lessons. And sometimes you learn a lesson too late and at that point you need to apologize because you were careless." Based on the lyrics, critics have speculated that the song is Swift's apology to her ex-boyfriend, The Twilight Saga actor Taylor Lautner. However, neither Swift nor Lautner has confirmed or denied these rumors. "Back to December" was first released as a promotional single from Speak Now on October 12, 2010 as an exclusive campaign by the iTunes Store leading up to the album release. On November 15, 2010, the song was then released as the second single from Speak Now.

"Back to December"'s accompanying music video was directed by Yoann Lemoine, who directed Katy Perry's "Teenage Dream." Swift's love interest in the video was played by male model, Guntars Asmanis. It premiered on January 13, 2011, on Country Music Television and Great American Country. It has been reported that parts of the video were filmed in Binghamton, New York, in MacArthur Park. Critical receptions towards the music video are mixed to positive. Jillian Mapes of Billboard believed that the video is "appropriately understated" considering the fact that the song is all about making an apology. Tamar Anitai of MTV described the video as "a bummer", writing "[It] isn't just about a girl with her calendar permanently stuck on the twelfth month of the year. It's not just about feeling like a Lady Antebellum song. It's about getting stuck in a place of romantic regret." In a different perspective, Kyle Anderson of MTV argued that the music video puts Swift in a "pantheon of modern classics" with regards to its "almost haunting visual sense" which makes up for the inadequate narrative. Leah Collins of Dose.ca called the music video as "dreamy and moody". Her points are echoed by Leah Greenblatt of Entertainment Weekly, who deemed the video as "kind of lovely", which corresponds with the melancholic yet regretful mood of the song.Joycelyn Vena of MTV noticed that the video is "darker" and "somber", something that is not synonymous with the Swift.

The video begins when a handsome young man is seen walking alone in a small town to the snowy football fields in a morning winter. A cut-scene shows Swift inside a house wearing a comfy sweater draped over one shoulder, brooding and singing about her lost love. About half way through, it starts to snow inside the spacious house. A montage of Swift writing a letter is shown alternately with scenes of her boyfriend strolling around the town. It is then revealed that the whole situation is the aftermath of a break-up between Swift and her boyfriend. It is also shown that Swift slips the letter that she has written in his coat pocket before he leaves. The final scene shows her bereft paramour sitting in the bleachers, reading her break-up letter.

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