Midrasch zu Wajikra 13:5

1) (Vayikra 13:5) ("And the Cohein shall see him on the seventh day. And if the plague-spot appears as it did before, if it did not spread in the skin, then the Cohein shall quarantine him for a second seven days.") "on the seventh day": I might think either in the daytime or at night. It is, therefore, written "on the day," and not at night.

2) I might think that all of the inspections of the day are kasher. It is, therefore, written (Vayikra 13:12): "to all the sight of the eyes of the Cohein." Just as with the Cohein — except when the light of his eyes has dimmed, so with the day — except when the light of the day has dimmed.

3) From here they ruled: Plague-spots are not inspected in the early morning or at twilight or inside the house or on a cloudy day, for then what is dull appears bright; and not at mid-day, for then what is bright appears dull. When are they inspected? On the third, fourth, fifth, seventh, eighth, and ninth hours. These are the words of R. Meir. R. Yehudah says: On the fourth, fifth, eighth, and ninth hours. R. Yossi says: On the fourth, fifth, ninth, and tenth hours, but I accept the words of the great one (R. Yehudah).