The Gemara in Megillah (5a-b) relates that one year on Purim, Rebbi Yehuda HaNassi planted a tree. After extensively discussing the legal ramifications and questioning the permissibility of doing so on Purim, one opinion in the Gemara suggests that he was allowed to do so because this particular tree represented a “planting of simcha” in the spirit of the joyful day. The Darkei Mussar questions why he felt the need to do so specifically on Purim. Why couldn’t he have waited for a more opportune and less hectic time to do so?
He writes in the name of his father-in-law, Rav Yosef Rozovsky, that every Yom Tov has a special lesson and power which may be acquired by one who properly taps into its latent potential. The lesson, such as freedom on Pesach and the ephemeral nature of earthly possessions on Succos, isn’t just for the duration of the Yom Tov, but it is to be absorbed and taken with us for the entire year.
Similarly, Rebbi Yehuda HaNassi chose to plant a tree specifically on Purim to symbolically hint that the joy and gladness we are intended to experience on Purim isn’t of the transitory drunken and frivolous variety. It should be a “planting” of a true deeper joy, one which develops from contemplating the miracles in the Megillah and results in a genuine trust in Hashem’s Providence and the accompanying sense of happiness and tranquility we will feel in our hearts throughout the year to come. (R’ Ozer Alport)