The Kedusha During Maariv & the Havdala Service

From B'yam Darkecha on the Beauty of Shabbos & the Yomim Tovim: R Yossi Michalowicz by R Yossi Michalowicz.

pp. 56 - 59: One can not begin his weekday activities until he brings the light of Shabbos into the weekdays, by saying the Kedusha which brings the Kedusha from the world of Atzilus into the world of Beriah. Havdala positions the weekday service in a proper way, by blending the concepts of "Ein Od Milvado" & Hashem's desire for an autonomous human bringing out the light from creation.