The moment a memory is entering one's brain that will, sometime in one's future, be remembered and repeated resulting in Deja Vu.
Billy went to school one day and saw something strange. He forgot all about it the next day. Ten years later he saw it again and said, "Man. I just got Deja Vu." Little did he know that this moment was foretold since the time he first had Duja Ve when he saw the strange object ten years before.