“The Moment I Knew” is the 17th track of the re-recorded album “Red” by Taylor Swift.
The song describes the feelings Swift felt when her boyfriend at the time, the actor Jake Gyllenhaal, did not arrive to her 21st birthday party.
In an interview with Swift by Yahoo Music she shared: "‘The Moment I Knew’ was a song about my 21st birthday party, which was the worst experience ever."

There are also some connection between this song to the 10 minute version of “All Too Well”, in which Swift expressed her feelings after her birthday party.