From Nookipedia, the Animal Crossing wiki
Totaka's Song is a song composed by Kazumi Totaka. The song appears in several games that Totaka composed the music for, including several Animal Crossing games and is hidden usually as an easter egg.
K.K. Slider's real name, "Totakeke," derives from Kazumi Totaka, a song composer for Nintendo. Totaka's Song appears in Animal Crossing as a hidden tune, in which the player must approach K.K. Slider when he plays music on Saturday night and request that he play the song K.K. Song, which is a version of Totaka's Song.
Animal Crossing (GCN) e-Reader Card Minigames
In the Series 4 e-Reader cards, cards P-13 and P-15 contain the "Who's Dunnit?" mini-game, which has Totaka's Song as its normal background music.
In Animal Crossing: City Folk, the player can find the song in the same way as the previous games, but there is also an alternative way to find the song. When Kapp'n is driving the player to and from the city, leaving the game running without clicking through his text will trigger Kapp'n' to whistle several of K.K. Slider's songs, including K.K. Song.
If the player waits about three or four minutes while riding the boat to Tortimer Island, Kapp'n can be heard whistling the song. This will happen only if there is considerable lag. The player can also just play K.K. Song, which is randomly mixed into the Club LOL DJ set.
Animal Crossing: New Leaf European website