Why does the narrator of "The Tell-Tale Heart" wait till the eight night to commit the murder?He watched him for 7 nights and on the eighth night...

In certain cultures, the number eight is extremely significant.  In the Jewish culture, for instance, the concept of man having the ability to transcend his nature is represented by the number eight.  When the High Priest officiated, he wore eight garments.  And, from their eighth day onward, animals could be offered as sacrifices.  In Edgar Allan Poe's story, "The Tell-Tale Heart," there seems much importance given to the eighth day by the narrator.  It is the day of completion. Poe's narrator writes,

Never before that night had I felt the extent of my own powers--of my sagacity.  I could scarcely contain my feelings of triumph. 

Not only does the narrator sense the significance of the eighth day, but the old man feels the power of this night as well.  He knows that the narrtor is going to kill him.