Hullin, Chapter Five, Mishnah Five
This mishnah, which I shall explain here and not below, teaches that when we reckon the day on which an animal was slaughtered, the night goes with the day that follows it. Thus if he slaughtered one animal at night and then the following day he slaughtered its mother or offspring, he has violated the law of it and its young. But if he slaughters an animal during the day and then the following night he slaughters its mother or young, he has not violated the law.
The basis for this law are the verses from Genesis 1 that state, and it was evening and it was morning, the first day, and so on for each day. These verses imply, to the rabbis, that the day begins with the preceding night. And since the words one day are used in the context of creation and in the context of the law concerning it and its young, the day is reckoned in the same way for each.
1) The one day mentioned in connection with the law of it and its young means the day and the night preceding it.
2) This was how Rabbi Shimon ben Zoma expounded (darash): it says one day (Genesis 1:5) in connection with the creation and it also says one day (Leviticus 22:28) in connection with it and its young Just as the one day mentioned in connection with the creation means the day and the night preceding it, so too the one day mentioned in connection with it and its young means the day and the night preceding it.