The day begins with alos hashachar (daybreak). Technically, one may begin the daytime reading of the Megilah at that time. Since the exact moment of alos is not always easy to ascertain (and thus a person might end up reading the Megilah while it is still night), Chazal instituted that the Megilah should only be read after sunrise. B’dieved, if a person read the Megilah once there was light across the eastern horizon, he is yotzei. Under pressing circumstances, a person can even read it l’chatchilah from alos hashachar and onward. The time for reading the Megilah by day extends until sunset. B’dieved, if one did not yet read the Megilah he can read it during bein hashmashos, but a brocha may not be recited.
[שו"ע תרפז, א, משנ"ב ד-ו, ושעה"צ ה; ביאורים ומוספים דרשו, 7-6]