The Shulchan Aruch rules that one should wear fine clothes when going to hear the Megilla. In fact the holy talmid of the AriZal, Reb Chaim Vital, would immerse in the mikveh and then put on Shabbos clothes in honor of Purim.
It is also written that after returning home from shul on Purim night, one should have the table set and candles lit, and partake of a seuda in honor of Purim.
(רמ"א סי' תרצ"ה ס"ב, כף החיים שם סקי"ג, רמ"א ס"א)
Throughout the month of Adar, the conduct of, Reb Shmuel Abba of Zichlin was very joyful, and on Purim his avoda expressed extraordinary simcha. He had harsh words for those who did not treat the day properly, or who did not dress in their Shabbos clothes in honor of the Megillah reading.
One year, his beis midrash was filled with all the townsmen who had come to hear the Megillah. All were dressed in their Shabbos best, except for one man, who wore his weekday garb. When some chassidim questioned his behavior, the man responded (playing on a common Yiddish folk-expression), “Purim is not a Yom-Tov and fever is not a sickness.” At that moment, the tzaddik entered, gave this fellow a piercing look and said, “Purim is a Yom-Tov, and fever is a sickness.”
As soon as he arrived home that night, this man suddenly fell ill with a raging fever. When it only worsened as the days wore on, he sent a message to the tzaddik, asking for a bracha.
The tzaddik replied, “Now he knows that fever is really a sickness, and he needs to know that Purim is really a Yom-Tov.”
The man suffered in sickness the entire year, until the following Purim.
(סיפו"ח זוין מועדים ע' 271, ובס' הצאצאים ע' 62 הביאו על אדה"ז)
