Megillah 20B

ספירה ביממא היא.

and counting can only be done during the day and not at night, as it says: “And she should count for herself 7 days” (Leviticus 15:28), she cannot immerse herself until after sunrise, although here she has to count only one day.

וכולן שעשו משעלה עמוד השחר כשר. מנהני מילי? אמר רבא: דאמר קרא: ״ויקרא אלהים לאור יום״, למאיר ובא קראו ״יום״.

§ The Mishnah concludes: And with regard to all these things, if one did them after daybreak they are valid. The Talmud asks: From where is this matter derived, that from daybreak it is already considered daytime? Rava said: As the verse states: “And God called the light [or] day” (Genesis 1:5), meaning: To that which was becoming lighter and lighter he called day. The Hebrew word or is not to be understood in its usual sense of light, but as a verbal noun: that which is becoming lighter and lighter. It teaches that as soon as light begins to appear in the sky it is called daytime.

אלא מעתה: ״ולחשך קרא לילה״ [למחשיך ובא קרא לילה]? הא קיימא לן דעד צאת הכוכבים לאו לילה הוא!

The Talmud raises a difficulty with this interpretation: However, if it is so that Rava’s interpretation of this phrase is correct, the following phrase: “And the darkness [ḥoshekh] He called night” (Genesis 1:5), should be interpreted in a similar fashion: That which was becoming darker and darker He called night, so that immediately after sunset it would be considered nighttime. But don’t we maintain that until the stars come out it is not nighttime? We are forced to say that ḥoshekh literally means darkness, and similarly, or in the first part of the verse literally means light.

אלא אמר רבי זירא, מהכא: ״ואנחנו עושים במלאכה וחצים מחזיקים ברמחים מעלות השחר עד צאת הכוכבים״, ואומר: ״(והיה) לנו הלילה (למשמר)״.

Rather, R' Zeira said: We derive this halakha from here, as it is stated: “So we labored in the work; and half of them held the spears from the rising of the morning till the stars appeared” (Nehemiah 4:15), where “rising of the morning” means daybreak, and the next verse states: “So that in the night they may be a guard to us; and labor in the day” (Nehemiah 4:16). This demonstrates that the day begins with the dawn.