And there was a thick darkness in all of Egypt for three days (Exod. 10:22)
The plague of darkness lasted for six days, three days of darkness when no one was able to see anyone else, and “a thick darkness… for three days” which was so dense that the Egyptians were unable to move. All of the other plagues (aside from the death of the first-born which lasted only one day) were seven days long. G-d saved the last day of darkness for when the Egyptians chased after the Jews in the desert. When the Jews travelled at night they were guided by a pillar of fire, but when the Egyptians chased them, G-d punished the
Egyptians by causing them to travel in darkness.
(Midrash Raba)
And G-d said to Moses and Aaron…”This month shall be for you the head of the months.” (Exod. 12:1-2)
The Jewish calendar follows the lunar system. The solar calendar is 365 days, and the lunar calendar is approximately 354 days. To make up for the deficiency, seven years in every 19 year cycle are leap years. Thus, in some years, the lunar year is actually a few days ahead of the solar year. The leap year serves as a reminder that everyone has an opportunity to make up for what he has failed to accomplish in the past.
Furthermore, just as the leap year not only makes up for the deficiency, but provides an “advance” on the future, a Jew must also intensify his efforts in his service of G-d and store up additional merits.
(The Lubavitcher Rebbe)
