This might be a tricky one: I've been trying to find out the name of a duet song for quite a long time. It is sung by a man and a woman and sounds like a love duet. The mood is rather melancholic, the song is slow, reminds me somehow of a rainy British day It might be because the singers sound British. Unfortunately I don't remember any lyrics, but as far as I remember they are singing about "on the first day", "on the second day" etc. Not quite sure though.
And it seems to be a rather popular song!