Megillah, Chapter Two, Mishnah 4. This Mishnah deals with two matters: who is qualified to read the Megillah and to fulfill the obligation on behalf of others, and which mitzvos are 'mitzvos of the day' that may be performed only after sunrise.
"Hakol kesherin likros es haMegillah":
The Mishnah opens by establishing that everyone is qualified to read the Megillah, "chutz mecheresh, shoteh, vekatan" - except for a deaf-mute, a mentally incompetent person, and a minor. These three are excluded because they are not obligated in the mitzvah:
Cheresh - one who is deaf and mute.
Shoteh - one who is not of sound mind.
Katan - one who has not yet reached the age of mitzvos.
Rabbi Yehudah, however, qualifies a minor. In his view, a minor may read and fulfill the obligation on behalf of others.
Mitzvos of the Day:
From here the Mishnah turns to discuss several mitzvos whose time is during the day, and a person does not fulfill his obligation in them until the sun has risen: "ein korin es haMegillah, velo molin, velo tovlin, velo mazin" - we do not read the Megillah, nor circumcise, nor immerse, nor sprinkle. These are the cases:
"Ein korin es haMegillah" - this refers to the daytime reading, not the nighttime reading.
"Velo molin" - a bris milah, whose time is during the day.
"Velo tovlin" - those immersions in a mikvah whose time is during the day.
"Velo mazin" - the sprinkling of the water containing the ashes of the red heifer, which is also performed during the day.
The Mishnah adds: "vechen shomeres yom keneged yom lo tivol" - and likewise a woman who watches a day for a day may not immerse. A shomeres yom keneged yom is a zavah who saw blood on one of the eleven days of zivah. Once she has seen blood on one day, the halachah requires her to observe the following day as a clean day; on that very day she immerses in a mikvah, and if she sees no further blood during the rest of the day, she is pure that night. She too may immerse only during the day.
The time for all of these is "ad shetanetz hachamah" - that is, ideally they should be performed only after sunrise.
After the Fact - From Daybreak:
Even though these mitzvos are ideally performed after sunrise, the Mishnah teaches us: "vechulan she'asu mishe'alah amud hashachar - kasher" - and all of them, if performed from the time that dawn broke, are valid. If, after the fact, a person performed one of these mitzvos - whether he immersed, or read the Megillah, or circumcised, or sprinkled from the ashes of the red heifer - after the light of morning appeared, that is, at daybreak, which precedes sunrise (and there are various opinions as to the precise interval between daybreak and sunrise), he has fulfilled his obligation.
It emerges that sunrise is the time that is certainly day, and ideally one should wait for it; but daybreak too is considered day after the fact, and all of these mitzvos performed from that hour onward are valid.
In summary: In this Mishnah we learned that everyone is qualified to read the Megillah except for a deaf-mute, a mentally incompetent person, and a minor, who are not obligated in the mitzvah (and Rabbi Yehudah qualifies a minor), and that the daytime reading of the Megillah, circumcision, immersion, and sprinkling - as well as the immersion of a shomeres yom keneged yom - are ideally performed from sunrise, while after the fact one fulfills the obligation in them from the time that dawn broke.