Haman thought that Adar was a month of bad luck for the Jews because of the death of Moshe and it was probably a painful and difficult day for the Jews being that they lost the greatest leader in history. But the Jews pulled it together and on that day they learned a very important lesson. They learned that as a people we must keep moving on and with the death comes a new beginning and opportunity. The death of Moshe was so important not only because of how essential of a leader he was but because the Jews response to his death showed us how we should react by every Jewish death.
The Gemara in Megilah 3a says that Haman didn’t know that he died and was born on the seventh of Adar. The Gemara is seemingly backwards as it should have said that Haman didn’t realize that he was born and died on the seventh, but instead it says that Haman didn’t know that he died and was born on that day. But it could be that Haman didn’t understand this idea that death is an opportunity to get to a place and level that wasn’t attainable during the life time of the individual. As the Gemara in Temurah 16b says the day Moshe died, 3,000 Halachos we forgotten and עתניאל בן קנז was mechadesh them through pilpul. This method would never have come to be if not for the death of Moshe. Therefore as painful as a Yahrzeit is in terms of the loss of the past, it also creates an opportunity for growth and in building a future.
This idea is also one of the messages of Purim and why we say עד דלא ידע בין ארור המן לברוך מרדכי because with all the events of Purim brought tremendous potential for growth so we are not even sure if it was a bad thing, everything that transpired, because of the growth that it brought.
In Taanis 29a, the Gemara says כשם שמשנכנס אב ממעטין בשמחה כך משנכנס אדר מרבין בשמחה. Regarding being ממעט בשמחה in Av, that is a Mishnah. But where did the Gemara see it to extend this to the month of Adar, and what is the connection-the כשם. But just like the month Av was a month of destruction, Adar is a time of rebuilding and from destruction comes rebirth. The same simcha we lose in Av can become a source of simcha because it enables growth that wasn’t possible when we were in the past. Just like we said earlier that Moshes passing created opportunity for pipul, so to the galus which came from the destruction of the Beis Hamikdash created a new opportunity for growth.
The Mechaber at the end of Hilchos Rosh Chodesh, סימן תכח, describes a סימן לקביעת המועדים through א"ת ב"ש. He writes that each day of Pesach corresponds to when a holiday during the year will fall out. For example, whatever the first day of Pesach is, Tisha Baav will be on that day of the week. It is understandable that all the holidays are נרמז in the holiday of Pesach as Pesach is in Nissan which is ראש חדשים. (Also as the Gemara says in Rosh Hashanah 4a that Pesach is the ראש לרגלים regarding the איסור of באל תאחר. In order to be עובר the איסור of באל תאחר it needs to be that three רגלים כסדרן pass starting with Pesach). But when it comes to the sixth day of Pesach, he writes that the sixth day of Pesach is the day of the week that the previous Purim was on. Why is it that the days of Pesach hint to all the holidays in the upcoming year but for Purim, it refers to the Purim from last year?
It must be that even though Nissan is technically the first month of the year, there is an element of Adar that makes it the beginning. Adar is at the end of the year, but the end is really the beginning (just like when a person hits rock bottom, he can only go up so it’s really the beginning for growth). Adar is אחרית כראשית because it is the month, with the holiday of Purim, which teaches us that an end can really be a new beginning. The end is already a beginning for the new beginning.
This message can be seen regarding Amalek. In Parshas Balak, the Pasuk says(כד,כ) ראשית גוים עמלק ואחריתו עדי אבד. Although Amalek flourishes in the beginning, but in the end, he withers. But the Jewish peoples greatness is that at the end, we don’t wither rather we renew ourselves and the end is really a new beginning.
The Chasam Sofer in Parshas B’haloscha says in the name of his Rebbe Reb Nosson Adler that the main growth of a Talmud only comes when his Rebbe passes away. As we see by Yisro that he says to Moshe לא אלך כי אם אל ארצי ואל מולדתי אלך(י,ל). Yisro was saying that if he stays by Moshe, then he won’t be a הולך and only once he is אל ארצי ואל מולדתי, away from Moshe, can he be הולך, because once he is away from his Rebbe, then he can shteig.
Rashi in Megilah on 4b says chapter 30 in Tehillim, מזמור שיר, is about Mordechai, Esther, and Haman. The end of the מזמור which says הפכת מספדי למחול לי is referring to Moshe’s death because although it was a tremendous מספד, it turned into a מחול because of all the potential future growth which was now possible as mentioned above. And this is the root of the miracle of Purim because that which Haman thought Adar was a bad month for the Jews because of Moshe’s death was really a good month because of the higher level of growth that were only able to be reached with his passing.