At sixes and sevens

Why must it called sixes and sevens ? why don't they just call this "sevens and eights" things ?
But whatever,I'm just wondering why. (lol)
After a crazy night,started to continue my work,this is my undone 3D project,I mistakenly thought that this week only is our submission week for this project,But in fact last week is already have to submit this work.But luckily,teacher didn't pursue for this,and give us another chance to hand it up next week.
And now,the "thing" is going to finished but not yet,hope I can finish it as soon as possible.