The morning after the wedding, Jacob discovered that the bride under the veil had been Leah, not Rachel (Genesis 29:25). The Targum Pseudo-Jonathan explains how the deception had been possible.
Jacob had apparently given Rachel private signs — tokens, sayings, maybe items of clothing — so that on the wedding night he would know his true bride even in the dark. Rachel, knowing her father's plan, made an astonishing choice. Rahel had delivered to her all the things with which Jakob had presented her.
Rachel handed her own signs to Leah.
Think about the cost. Rachel had waited seven years. She had watched Jacob serve her father for seven years of her life. She was the bride he had worked for. And on the wedding night, she quietly transferred the keys of her own identity to her older sister so that Leah would not be humiliated under the canopy. Rachel chose her sister's dignity over her own wedding.
When Jacob saw Leah in the morning light, the Targum reports his stunned confrontation: What is this that thou hast done to me? Was it not for Rahel that I served with thee? Why hast thou deceived me? His rage is directed at Laban, not Leah — because Jacob understood quickly that Leah had not orchestrated this. Leah had been placed in the bed by her father.
Later tradition (Bava Batra 123a) says that Rachel's sacrifice was remembered in heaven for centuries. When Israel was exiled and the patriarchs pleaded for mercy, it was Rachel's tears — the tears of the woman who had given up her wedding night — that moved God to promise the return (Jeremiah 31:15–17).
The takeaway: Rachel's greatest act was not marrying Jacob. It was the moment she let her sister marry him first.