Said 21 times in Bible. One is from dedication of Temple. Pulled from the blueletter bible.
“Likewise you shall do with your oxen and your sheep. It shall be with its mother seven days; on the eighth day you shall give it to Me.
It came to pass on the eighth day that Moses called Aaron and his sons and the elders of Israel.
“And on the eighth day he shall take two male lambs without blemish, one ewe lamb of the first year without blemish, three-tenths of an ephah of fine flour mixed with oil as a grain offering, and one log of oil.
“He shall bring them to the priest on the eighth day for his cleansing, to the door of the tabernacle of meeting, before the LORD .
‘On the eighth day he shall take for himself two turtledoves or two young pigeons, and come before the LORD , to the door of the tabernacle of meeting, and give them to the priest.
‘And on the eighth day she shall take for herself two turtledoves or two young pigeons, and bring them to the priest, to the door of the tabernacle of meeting.
“When a bull or a sheep or a goat is born, it shall be seven days with its mother; and from the eighth day and thereafter it shall be accepted as an offering made by fire to the LORD .
‘For seven days you shall offer an offering made by fire to the LORD . On the eighth day you shall have a holy convocation, and you shall offer an offering made by fire to the LORD . It is a sacred assembly, and you shall do no customary work on it.
‘Also on the fifteenth day of the seventh month, when you have gathered in the fruit of the land, you shall keep the feast of the LORD for seven days; on the first day there shall be a sabbath-rest, and on the eighth day a sabbath-rest.
‘Then on the eighth day he shall bring two turtledoves or two young pigeons to the priest, to the door of the tabernacle of meeting;
On the eighth day Gamaliel the son of Pedahzur, leader of the children of Manasseh, presented an offering.
‘On the eighth day you shall have a sacred assembly. You shall do no customary work.
On the eighth day he sent the people away; and they blessed the king, and went to their tents joyful and glad of heart for all the good that the LORD had done for His servant David, and for Israel His people.
And on the eighth day they held a sacred assembly, for they observed the dedication of the altar seven days, and the feast seven days.
Now they began to sanctify on the first day of the first month, and on the eighth day of the month they came to the vestibule of the LORD . So they sanctified the house of the LORD in eight days, and on the sixteenth day of the first month they finished.
Also day by day, from the first day until the last day, he read from the Book of the Law of God. And they kept the feast seven days; and on the eighth day there was a sacred assembly, according to the prescribed manner.
‘When these days are over it shall be, on the eighth day and thereafter, that the priests shall offer your burnt offerings and your peace offerings on the altar; and I will accept you,’ says the Lord GOD.”
So it was, on the eighth day, that they came to circumcise the child; and they would have called him by the name of his father, Zacharias.
“Then He gave him the covenant of circumcision; and so Abraham begot Isaac and circumcised him on the eighth day; and Isaac begot Jacob, and Jacob begot the twelve patriarchs.