Midrasch zu Wajikra 15:14

12) (Vayikra 15:14) ("And on the eighth day, he shall take for himself two turtle-doves or two young pigeons, and he shall come before the L–rd to the door of the tent of meeting, and he shall give them to the Cohein.") "on the eighth": I might think either in the daytime or at night; it is, therefore, written "on the day" — on the day and not at night.

13) "he shall take for himself." If he separated them for his first zivah, he should not bring them for his second zivah. "two turtle-doves or two young pigeons": The exchange of turtle-doves are young pigeons and the exchange of pigeons are turtle-doves. (i.e., If he set aside money for the first, but could not find them, he may bring the second, and vice versa.) But their exchange is not the tenth of an ephah.

14) Where is it derived that he immerses (on the seventh day) when it is yet day (and not when it gets dark)? From "and he shall come before the L–rd to the door of the tent of meeting." How could he come to the door of the tent of meeting unless the sun had gone down? We are hereby taught that he immerses when it is yet day.