Purim is considered a minor holiday on the Jewish religious calendar, which means that it is not attended with some of the same halakhic restrictions observed on other holidays. The primary commandment of Purim is to recite three blessings before hearing the scroll of Esther(Megillat Ester) recited:
Blessed art Thou, LORD our God, Master of the universe, who sanctified us with His commandments and commanded us regarding the reading of the Megillah. Amen.
After these blessings are recited, the scroll of Esther(Megillat Ester) is read. At the end of the reading, an additional blessing regarding God's vengeance is often recited - and then the oneg begins!