Young Goodman Brown feels that he must test the power of good, so he attends the Black Mass one night. This, in fact, is the night on which he must "tarry away" from his new bride, Faith. Certainly, the symbolism of her name cannot be lost upon the reader of Hawthorne's story.
This statement also underscores the theme of Guilt vs. Innoncence and Good vs. Evil in "Young Goodman Brown." For, Goodman in his religious complacency believes that he can traverse the forest with the devil, encounter hypocrites such as Goody Cloyse and Deacon Gookin (real characters involved in the Salem Witchcraft Trials of 1692) and return spiritually unscathed.
However, while his wife takes the "leap of faith" and welcomes Goodman home the next day, Goodman proves his statement that he must "tarry away from you" to be ironic. For, after his traveling between good and evil,--"twixt now and sunrise"--he no longer believes in his wife's innocence or the prevailing innocence of the Puritans in his community; he has, indeed, moved away from his faith. Henceforth, he now believes what the "sable form" of the devil has told him: "Evil is the nature of mankind."
A stern, a sad, a darkly meditative, a distrustful, if not a desperate man did he become from the night of that fearful dream.
Critics often view this shrinking "from the bosom of Faith" of Young Goodman Brown as an indictment by Hawthorne of the unredemptive nature of Puritanism, a belief in the evil nature of human beings because of original sin, a sin that damns most people for eternity.
At some point in our lives, all of us must leave our "F/faith" (innocence/naieve understanding of the world) and go on a journey into "darkness" /ambiguous reality ("twixt now and sunrise"). On this journey we learn, among other things, that all is not as we thought. This is particularly true in Brown's case of the people he knew and held in unrealistic/naieve esteem. He thought they were angels; they were only people. Of course, we don't know if any of the people in the story really were that horrible; the story seems to suggest that they were not, although there are realities mentioned that are horrible such as being the only witness as a maiden buries her child.
These "realities" are part of life; things are far from perfect. Without real evidence of anyone's failings (remember that the night is as likely a dream as a "reality"), Brown judges them for being less perfect than he needed/wanted them to be. Instead of growing in his F/faith, he becomes paralyzed in an infantile perception of what reality should be.
We all have to make this journed as we realize that those we held in high esteem, while not being evil, certainly are fallen. How we deal with it determines a great deal about how we deal with "reality."
The enemy of the good is the perfect.
he means that he needs to be away from Faith, his wife, this night. 'of all nights in the year' means perhaps that he is home most other nights and that tonight is significant because he has important business to do away from home. for example if i were meeting someone important i might say 'of all the nights this year, tonight i have to be away from the house.' 'must i tarry away from thee' means I must be away from you. the journey needs to be in darkness literally because his business requires him to arrive in the morning and symbolically because darkness and the forest are archetypal settings of the devil in hawthorne's and others' books.