Why do we make a Seuda at Purim? Why do we get dressed with customs at Purim?
ב"ה Shalom, I received your question only a few days ago, although we are busy now with Pessach. It says in the Megilla (9:22) "Days of feasts and joy", so from here are Rabbis inferred making a feast on Purim. Regarding getting dressed up with costumes, the Rema (696:8) in the Shulchan Aruch mentions that when masquerading people have to conduct themselves in a certain way as a given fact without giving a source. One of the reasons, I heard many years ago is because G-d himself was masqueraded as it were. Meaning, G-d did not do any supernatural miracles in the story of Purim, but the deliverance came from Hashem without Him being "seen". Therefore, we similarly masquerade ourselves to commemorate this. Another reason offered, is that since a person is supposed to be so happy Purim and appreciate all G-d's lovingkindness to such a degree that he won't be able to differentiate between "Cursed is Haman and blessed is Mordechai " .Similarly, when we are masqueraded no distinction can be made and we are thereby praising Hashem in another way. All the best.