חייב אדם לקרו' המגילה בלילה ולחזור ולשנותה ביום ושל לילה זמנה כל הלילה ושל יום זמנה כל היום מהנץ החמה עד סוף היום ואם קראה משעלה עמוד השחר יצא:

One is obligated to read the megillah at night and return and repeat it during the day. And for [the reading] at night - its time is all night, and for [the reading] during the day - its time is all day, from sunrise until the end of the day; and if one read it from daybreak, he has fulfilled [his obligation.]

We cancel Torah learning to hear the megillah, all the more so for the rest of the mitzvot of the Torah, that all of them are pushed aside because of the reading of the megillah, for there isn't anything that pushes aside megillah reading, except for an unclaimed dead body that does not have anyone to bury it (according to its need), in that the one who happens upon it buries it first and afterwards reads [the megillah]. Rema: And all of this is only dealing with [a situation] in which one is able to do the two of them. But if it is impossible to do the two of them, there is no Torah mitzvah that is pushed aside because of megillah reading (Ran and Beit Yosef in the name of the Tosafot and Mehar''a Mizrachi). And the fact that an unclaimed dead body comes first, this is specifically when it is possible for him to read afterwards (Mehar''a Mizrachi).