According to the Talmud Yerushalmi (Megillah 1:4), the verse stresses that the holiday would be celebrated “every year and year” to demonstrate that Purim will never be abolished, even in the time of Moshiach.
R’ Yaakov Emden points out that these days were already established as days of joy. After all, Moshe Rabbeinu was born on the seventh of Adar, so his bris (“circumcision”) should have been scheduled for the 14th. However, the Talmud (Sotah 12a) says that he was born circumcised, so he only required a symbolic pin-prick called hatafas dam bris (Shulchan Aruch Yoreh Deah 262:1 and 264:1), and this procedure is not held on a Shabbos (Shulchan Aruch Yoreh Deah 260:2 and 263:1). Therefore, Moshe’s bris was on the 15th of Adar.