The Rebbe taught this discourse on Purim in 1965.  It focuses on the verse (Esther 6:1) 'On that night the sleep of the King was disturbed'.  This is the point in the story when Ahasuerus cannot sleep and has the chronicles of his kingdom read to him, and he discovers that Mordechai had saved him fro m being assassinated, but had never been rewarded.  At that moment Haman walked into the King's chamber seeking permission to have Mordechai hanged.  The King asked 'what should be done to the man whom the King wishes to honour?'.  

Haman, thinking he was the one whom the King intended to honour, gave an extravagant answer, and then was horrified to hear it was Mordechai who had to be honoured in that way, with Haman himself acting as a herald announcing 'This is the man whom the King wishes to honour'.  At this point in the story the plot changes from dark to light, from sorrow to joy.  The discourse explains that the King whose 'sleep' is disturbed is the Holy One, who is responding to the fact that the Jews identified as Jews despite the pressure to conform and worship Haman as an idol.  'Night' (contrasting with 'day') represents concealment.  In the depths of concealment, the depths of Exile, the Divine was revealed, bringing a joyful turnabout for the Jews.