The Daily Ma’amar of our Rebbe Rav Eliezer Berland shlit”a, courtesy of the “Hashem Hu HaElokim” leaflet:
The Secret of “Seder Night, Seven Days of Passover, and the Shavuot Holiday
Seder Night begins with the eating of the Karpas, the secret of a “green line which creates new things,” the secret of “Esther was of a greenish complexion.” And then on Seder Night, through three matzot and four cups, the serpent with seven heads is subdued. And through the removal of the serpent with seven heads is the holiness of Passover and the exodus from Egypt.
Because the Mixed Multitude are the serpent with seven heads, and Moshe mistakenly drew them close, and they caused the Jewish people to err and seduced them 40 days after the giving of the Torah to make the calf. And if it hadn’t been for them, the “Maor VaShemesh” says, they [the Jewish people] would have immediately entered the Land of Israel, and would not have needed the miracle of the splitting of the Red Sea, the manna, and the quail.
For then, at the conclusion of the seven days of Passover, through the splitting of the Red Sea, the serpent with seven heads is completely subdued, so that it will not have power throughout the year until the next Passover. However, then, the Jewish people only merited to the outer aspect of Yechida, and they needed another fifty days to reach the level of the giving of the Torah, and then they merited to the inner aspect of Yechida, and Moshe opened up for them all seven firmaments and seven levels of Gehinnom, and showed them that Hashem is God in the Heavens above and on the earth below, there is no other (Devarim 4:39).
And then, on the night of Shavuot, this is the illumination of Atik [the Ancient of Days] — the inner aspect of Yechida, and we still need to reach — through King Shlomo who composed the Song of Songs, and through this he built the Temple, because Shlomo merited to reach the inner aspect of Adam Kadmon, and therefore in his days, the weekdays were an aspect of Shabbat.
Because then in the days of Shlomo, even on weekdays the inner aspect of Adam Kadmon shined. And even in the Second Temple, place and time were nullified, and women from Tzippori would fly every morning to the Temple and return before the gathering of the figs in order to send all the children on time to Heder.