Parah, Chapter 12, Mishnah 11. This chapter deals with the practical details of the sprinkling of the mei chatas, the waters of the parah adumah. Our mishnah, the closing mishnah of the masechta, teaches that the dipping of the ezov and the sprinkling itself both belong to the daytime, and it clarifies exactly when the day begins for this purpose.
Dipping and sprinkling must both be by day
"Taval ha'ezov bayom": he took the ezov and dipped it into the mei chatas while it was still day, "vehiza bayom", and he also performed the sprinkling while it was still day. Of such a case the mishnah rules "kasher": the sprinkling counts.
Now the opposite cases. "Bayom vehiza balaylah": the dipping was done while it was still day, but the sprinkling was delayed until after dark. "Balaylah vehiza bayom": the dipping was done in the night, and only afterward, in the daytime, did he sprinkle. In both instances the ruling is "pasul". Each half of the procedure, the dipping no less than the sprinkling, must fall within the daylight hours, and getting one of the two right does not rescue the other.
The immersion of the person himself
"Aval hu atzmo": the man undergoing purification is a different matter. "Tovel balaylah": he is permitted to enter the mikveh while it is still night, "umazeh bayom", and the sprinkling is then done on him after daybreak. His tevilah is not one of the components of the sprinkling that the mishnah confined to the day. Besides, his immersion only takes full effect once the following morning has come, since a night has to pass over him.
"She'ein mazin ad shetanetz hachamah." The reason the sprinkling itself waits for the morning is that one does not sprinkle until the sun has actually risen.
From dawn onward
"Vekhulan she'asu": all the acts spoken of here, if a person went ahead and did them "mishe'alah amud hashachar", from the moment that the column of dawn light rose in the east, are ruled "kasher" after the fact. The proper practice is to hold off until the sun is up, but the appearance of amud hashachar is already enough that what was done is not thrown out, even though the full day has not yet begun.
With this halachah we complete Masechta Parah.