EXOD197 It was taught: "There will not depart, the pillar of cloud, by day, and the pillar of fire, by night"- This teaches us that the pillar of cloud overlapped [in the transition] the pillar of fire, and the pillar of fire overlapped the pillar of cloud (Shabbat 23b). Scripture hereby teaches us proper deportment vis-à-vis the eve of Sabbath [namely, to add from the non--consecrated (i.e. Friday afternoon) to the consecrated (Sabbath). While the pillar of cloud was still in place, the pillar of fire emerged. (Mechilta)