New Defender's Study Bible Notes
4:3 process of time. Literally, “at the end of the days,” undoubtedly a reference to the “seventh day,” which God had hallowed as a day of rest and blessing (Genesis 2:3). On such a day, men would follow God’s example in ceasing from their regular labors in order to have fellowship with God, possibly meeting with Him at the entrance to Eden (Genesis 3:24).
4:3 an offering. Such fellowship, however, required that worshipers approach God with an offering that would make them suitable for His presence. Adam and Eve no doubt had instructed their sons that this required a substitutionary sacrifice of innocent blood (Genesis 3:21). Cain, however, chose to bring another type of offering on this occasion.