Lyric discussion by georginz 

Cover art for Birthday lyrics by Kings of Leon

As mentioned, I think the song is self explanatory, a guy and a girl going home after some drinks to continue celebrations. It reminds me of 'Pick up Truck' in the way he walks the girl home and they really like each other. Maybe "Pick up truck" is the sequel to this song, and this one explains a bit about the two of them at home before the other dude arrives.