The Torah simply says Cain brought an offering "in the course of time." Targum Pseudo-Jonathan on (Genesis 4:3) fixes a date: the fourteenth of Nisan.
That is the eve of Passover. The Targumist is planting a calendrical marker. The first offering ever brought by a human being was brought on the day that will one day become the eve of the Exodus, the night of the Paschal lamb. Jewish time, the Targumist suggests, has always cycled back to this date.
And what did Cain bring? "Of the produce of the earth, the seed of cotton (or flax), an oblation of first things before the Lord." Some of the earliest midrashic readings identify Cain's offering as the poorest, lowest-quality grain — leftovers rather than first fruits. The Targum's detail about "seed of cotton" (flax) may suggest something non-edible, a token offering rather than a real one. The first act of sacrifice in human history is already flawed at its root.