In America today, we have no practical applications for Shushan Purim, except for some changes in our tefillot.
But there is a Bitachon lesson for us in this day. We just need a history refresher to appreciate it.
In the days of the Megillah, on the 13th of Adar, all the Jews in all cities (non-walled and walled, including Shushan) were permitted to fight their enemies. 75,000 Nazis of Amalek were killed on that day. The next day, the 14th of Adar, the Jewish people in cities all over celebrated this miraculous victory, except the Jews in Shushan - they needed an extra day to fight because there were more enemies from Amalek in Shushan.
So, on the 14th of Adar that year, there were world-wide celebrations everywhere except in Shushan, because they were still fighting there.
The next year, the Megillah tells us, and for a few years thereafter, on the 14th of Adar, the Jews in non-walled cities commemorated their miraculous victory and celebrated. But the Jews in walled cities didn’t partake in these celebrations, because, as Ramban says in Megillah, they felt that due to their walls, they hadn’t really been in danger. They had spontaneously celebrated the first year but didn’t think their victory was worthy of a fixed commemoration every year. They felt that due to their protective walls, they would’ve been ok.
The Gemara teaches that whenever there is a miraculous salvation, whether for an individual or a community, there should be an annual commemoration of the event. We can see this in the case of Ness Mussan, a miraculous salvation from a blood libel that occurred in Aleppo on the 13th of Sivan many years ago (for details of this event see Aleppo City of Scholars). We continue to acknowledge that miracle till today by skipping Tahanun on that day (see Magen Avraham siddur)
If a miracle occurs, you are supposed to commemorate it every year. So, while non-walled cities felt it was a great miracle and continued to celebrate year after year, the walled cities did not.
A few years later Mordechai was divinely inspired to make the holiday official, and directed the non-walled cities to continue to celebrate on the 14th of Adar. The city of Shushan was directed to celebrate on the 15th, as they had done previously. The people in the walled cities, however had their celebrations put off until the 15th as well. Although technically their salvation came on the 14th, their celebration was pushed off to the 15th as a demonstration of Hashem’s displeasure with their outlook. Hashem was telling them: Just because you were in a walled city you thought you were safe? Normally holidays are not pushed off, but those in walled cities’ lacked the recognition that it was a true miracle, so their holiday was pushed off a day.
This is our Bitachon lesson- Don’t think that it was the walls of the city that kept you safe.
Sometimes when there is financial difficulty, such as turmoil in the market, a person with money isn’t concerned. He thinks he’s safe because of his resources.
A person who got a kidney might make a party every year for that, - but the small things might not be acknowledged. This is not to say we have to make a party for every good thing that happens, but we do have to realize it’s all coming from Hashem.
Shushan Purim is our lesson: Don’t think that because you have a wall you’re safe. That’s a mistake and it applies to us today as well. Even if you feel you’re safe, you’re only safe if you rely on Hashem.