The Mishnah states that one should perform certain mitzvoth (including reading the Megillah) after the sun has already risen. But it added that if one performed any of them after dawn, meaning after it was already a bit light outside but the sun had not yet risen, the act is valid. Our sugya comments on the source of this halakhah.
Rava proves that day mitzvoth may be performed after dawn and before sunrise from the first day of creation. God called the light “day” even though there was no sun—it wasn’t created until the fourth day. Thus even now that we have a sun, the light that appears in the sky before the sun is a sign of day.
The problem with Rava’s statement is that it would imply that night could begin without the stars. After all, God calls the darkness night even without the stars.
Zera proves that day begins at dawn from two verses in Nehemiah. In v. 15 we learn that while the workers were working, the guards watched over them from the rising of the morning (dawn) until the stars appeared. In v. 16 we see that these are the parameters of day—it begins at dawn and ends when the stars come up. Thus if any of these mitzvoth are performed after dawn, the mitzvah is still valid.
Today's section consists of the next two mishnayot. My commentary is taken fromMishnah Yomit. One can fulfill one’s obligation of hearing the Megillah at any time during the day. Our mishnah gives a very long list of numerous other rituals that one can perform at any time during the day. Since this list is very long, I will not explain each item in detail, but rather mostly make reference to the relevant biblical verse(s) which deal with the issue. I will not even comment on issues that seem abundantly clear.
Sections 6-7: On Shabbat, holidays and Rosh Hodesh there are musaf sacrifices and musaf prayers. The sacrifices can be offered at any time during the day and the prayers may be recited throughout the whole day.
Section 8: For confession over the ox—One who brings an ox as a sacrifice also confesses to the sin for which the ox is brought (Leviticus 4:3,14).
Section 9: For the confession over the tithe—On the fourth and seventh years of the sabbatical cycle one makes a confession that he has “removed all of the holy produce from his home” (Deuteronomy 26:13).
Section 10: For the confession of sins on Yom HaKippurim—The high priest recites a confession over the sacrifices (Leviticus 16:21; see also Yoma 3:8, 4:2, 6:2).
Section 11: For laying on of hands—one lays one’s hands on an animal sacrifice before it is slaughtered (Leviticus 1:4).
Section 13: For waving [them]—one waves the innards and the breast of wellbeing offerings (Leviticus 7:30).
Section 14-16: these are all elements of the minhah offering.
Section 17: For pinching off [the head of a bird-offering]—Leviticus 1:15, 5:8.
Section 18: For receiving the blood [in a vessel]—in order to sprinkle it on the altar.
Section 20: For breaking the neck of the heifer—if a dead body is found in a field and the identity of the murderer is unknown (Deuteronomy 21:1)
Section 21: And for purifying the metzora—Leviticus 14.
Today's section provides proof texts for why the mitzvoth we learned in yesterday's section must be performed during the day.
I think that most of these are pretty self-explanatory. As long as the Torah/Psalms uses the word "day" the mitzvah must be performed during the day. The musaf prayer is obviously not mentioned in the Tanakh—no fixed prayers are mentioned in the Tanakh. But it must be performed during the day because it is like the musaf, additional sacrifices.
The confession that the High Priest makes over the oxen on Yom Kippur must be during the day because it is analogized to the other confession he makes on Yom Kippur, which is just words. There it explicitly mentions "day" so all of the confessions must be by day.
All of these should be pretty clear as well. Yesterday's section explained what all of these actions were. Today the Talmud explains how we know that they have to be performed during the day.
This is sort of a catch all verse used to prove that the rest of the actions performed in the sacrificial service must be performed during the day.
The rule that the sotah (suspected adulteress) ritual must be performed during the day is derived from the use of the word Torah in that context and in the context of judgment. Since we know that court cases can be tried only during the day, we can conclude that the sotah ritual must also be carried out during the day.