One must read the Megilla at night and again during the day, to commemorate the fact that the Jews cried out to God in their time of need during the day and at night (Megilla 4a; Rashi ad loc.).
The nighttime Megilla reading may take place at any point during the night – from tzeit to alot ha-shaĥar – while the daytime reading may take place an time during the day – from sunrise (and be-di’avad, from alot ha-shaĥar) until shki’a. However, the zealous perform mitzvot promptly, reading the Megilla at night immediately after Ma’ariv and during the day immediately after Shaĥarit (sa 687:1, 693:1, 693:4).
One may not eat or sleep before reading the Megilla at night. Studying Torah, however, is permissible. One who finds it difficult to extend Ta’anit Esther until after the Megilla reading may drink beforehand, on condition that he avoids intoxicating drinks. Similarly, one who is very hungry may eat a snack before Megilla reading. That is, he may eat as much fruit as he wants and up to an egg’s volume (kebeitza) of grain-based (mezonot) foods (sa 232:3, mb ad loc. 35; Rema 692:4, ma ad loc. 7, mb ad loc. 14-15).
The same laws apply to the daytime reading. However, since this reading takes place immediately after Shaĥarit, one must also be careful about all the prohibitions that apply prior to Shaĥarit, which are more stringent. Nonetheless, one who already prayed Shaĥarit and has yet to hear the Megilla should not eat before fulfilling the mitzva. Under pressing circumstances, however, one may eat a snack before hearing the Megilla. Likewise, a woman may not eat before hearing the Megilla. If she is very hungry, she may eat a snack, but not a full meal (mb 692:15-16; in a time of very great need, she may ask someone to remind her to hear the Megilla, and then she may eat a full meal before the reading).
Some of the greatest Rishonim maintain that the mitzva of reading the Megilla and publicizing the miracle is primarily fulfilled during the day, like all the other mitzvot of Purim. Therefore, one must be more meticulous about the daytime reading and make an even greater effort to read it in the presence of a large group of people, or at least a minyan.