Question: "How could Jacob not notice he married Leah instead of Rachel?"
Answer: Jacob was fleeing from his brother, Esau, who wanted to kill him, when he met Rachel. He was looking for his uncle, Laban, and asking about him at a well when Rachel, Laban's younger daughter, came to water her flock. Jacob told Rachel who he was, and she ran to tell her father. Laban offered Jacob a place to stay, and Jacob stayed. After a month, Laban offered to pay Jacob for his work. Laban had two daughters; Rachel was the younger of the two and "had a lovely figure and was beautiful. Jacob was in love with Rachel" (Genesis 29:17–18). Jacob had no other means to offer a bride price, so he offered to work for seven years in exchange for Rachel. When the seven years ended, Laban threw a feast for the wedding. However, instead of Rachel, Laban sent his older daughter Leah to Jacob, and Jacob had relations with Leah. Jacob’s shock at Laban’s deception is palpable the next morning: “When morning came, there was Leah!” (Genesis 29:25). How could Jacob possibly not have noticed that he was marrying Leah instead of Rachel?
Darkness could have had something to do with it. Genesis 29:23 says that, “when evening came,” Laban brought Leah “to Jacob, and Jacob made love to her.” Laban could have purposefully waited until it was dark to bring Leah to Jacob because he wanted to make sure they could not see each other. Or it may have been the custom for the consummation of marriage not to occur until after the feast or until late evening. Either way, it was dark, a fact that the Bible mentions. A heavy veil on Leah and ornate bridal clothing would also have aided the deception and may explain how Jacob did not notice that he married Leah instead of Rachel. It is also possible that Leah and Rachel looked similar and were generally the same size, and that this, in conjunction with it being dark and Leah being veiled, aided in the deception.
Other possibilities are not explicitly found in the text but deserve consideration, too. One possibility is that Jacob might have been drinking, and that impacted his perception. It was customary for feasts, especially wedding feasts, to include alcohol. We do not know for certain if this feast had intoxicating drink, nor do we know if Jacob was inebriated. However, it is possible, and Jacob being drunk would have impacted his perception.
Another possibility is that Jacob and Leah did not talk at all during their night together. Years earlier, when Jacob had tricked his father, Isaac recognized Jacob’s voice (Genesis 27:22). On Jacob’s wedding night, assuming Jacob had previously talked with Leah, he would have recognized her voice. Therefore, it is possible that Leah kept quiet or no words were exchanged that night.
It is also possible that Jacob did not get to know either Rachel or Leah during those seven years. In that culture couples did not “date”; marriages were arranged, and Jacob had arranged for Rachel to be his wife. Getting to know her would come after the wedding. The Bible says that “Rachel had a lovely figure and was beautiful” (Genesis 29:17), but nothing else is mentioned as to why Jacob loved her. So, Jacob’s infatuation with Rachel may have not resulted in actually talking much to her—or to her sister—before the wedding.
It is also possible that Jacob was not the only one Laban fooled. It could be that Leah and Rachel were fooled as well. Perhaps Laban did not tell his daughters about his arrangement with Jacob (Genesis 29:18), and Leah assumed that, according to custom, she (as the older daughter) was to marry Jacob that night (see verse 26). Imagine Leah’s shock and sadness when Jacob wakes up upset about having married her and then reveals to her that he had been working seven years for her sister (Genesis 29:25)!
Jacob stayed married to Leah, but a week later he also took Rachel as his wife—agreeing to work an additional seven years for her (Genesis 29:27–28). In the end, we cannot be certain of why Jacob failed to notice that he had married Leah. Scripture mentions only the darkness. We do know that Jacob did not have physical relations with either Leah or Rachel before that night. And we know that, just as Jacob had tricked his father, Isaac, so Laban tricked Jacob.