One of Hawthorne's themes in "Young Goodman Brown" is how the Puritan belief system affects the behavior of its believers. Among other things, Puritans such as Brown and his wife, Faith, believe that Satan can be a physical presence in their lives in addition to being a spiritual being. Puritans also believed, for example, that Satan could and would attack their faith while they dream in order to lead them astray. When Faith realizes that she cannot stop Brown from taking this journey, she says,
. . . put off your journey until sunrise and sleep in your own bed tonight. A lone woman is troubled with such dreams and such thoughts that she's afeared of herself sometimes. Pray tarry with me this night, dear husband, of all nights in the year.
Faith's warning is meant to remind Goodman Brown of several important consequences of his journey: first, because they both believe that Satan can appear to people in the form of dreams--a time when their will is the weakest--Faith reminds Brown that she is most vulnerable during the night while she is dreaming and that his presence might help protect her; second, from practical standpoint, going into the forest at night, which is a very dangerous place for any English villager, would be safer during the day; third, Faith's comment about "of all the nights in the year" may indicate that Brown is leaving on All Hallows Eve (Halloween), the night when the veil between the living and dead is traditionally thought to be the thinnest and a night when Satan is strongest, reminds him of not only his danger but also hers, as well. In other words, Faith pleads with Brown not to go because she feels herself to be vulnerable to Satan's influence, and this fear plants the seed in Brown's mind that grows into his belief that Faith has given herself to Satan when he dreams that her pink ribbon falls out of the sky while Brown is in the forest.
Brown, of course, because he is determined to explore the dark side of his nature, tells Faith not to worry and reminds her that his journey "must needs be done 'twixt now and sunrise." His journey, then, most likely takes place on Halloween because Brown recognizes that he is likely to encounter what he seeks--Satan--on this night. The fact that he ignores Faith's concerns indicates his complete faith in Faith, which actually foreshadows what happens to him during his dream vision--his complete loss of his religious faith and his loss of faith in his wife.
Faith's attempt, then, to stop Goodman Brown establishes two very important aspects of Puritan beliefs--Satan can attack while the faithful are sleeping and the forest itself is dangerous.