While Haman described the Jewish nation as "scattered and dispersed" (Esther 3:8), Esther urged the Jews to "assemble together" (4:16).  Intuitively she understood that the antidote to Haman's diabolical plot was Jewish unity. 

The celebration of Purim is followed by an ancillary holiday, Shushan Purim, observed only in cities with walls from antiquity (i.e., Joshua's conquest of Canaan in 1300 BCE). The holiday marks the experience of the Jews of Shushan and how, following their added day of battle, they celebrated one day later.

While to us Shushan Purim may seem like an anticlimactic footnote, Rabbi Shmuel Bornsztain of Sochatchov (1855-1926) argues that the power of Shushan Purim eclipses Purim itself.  How so?  In life, it's easy to celebrate our own personal salvation.  To even notice the triumph of others is much more difficult.    True joy is marked by our ability to connect and share with those around us.  Real happiness is never self-centred.  Shushan Purim is the day on our calendar where we share the joy of others and thereby enhance ours as well.  Let's make today the happiest day of the year!