This was a total quickie OTLSong is Someday by The Strokes, one of my favorite bands ;;
Firstly, there are two different versions of this song. The original was a piano piece written by Sakamoto Ryuichi for a movie of the same name. The second is a sample of the original piece, with added lyrics, found on Utada Hikaru's latest English album, This Is The One. My inspiration for this came from the latter, though I really love the original as well.

Day 5 ... The song is 5iVE STAR by capsule, a Shibuya-kei Japanese group. Cute song.
Day 3, a few hours late, but I figure it still counts because I'm awake haha!