The eighth day is a reference to the eighth day that Aaron and his sons were consecrated as priests. It is not a reference to the eighth day of the month. Seven days of cleansing is a typical time period and was commanded at the end of the previous chapter
The laws of the guilt offering are similar to the laws of the sin offering of chapter six.
The first five chapters in Leviticus detailed the mechanics of the five offerings. These next two chapters emphasize the ritual of the sacrifices. They detail the care the priests must take in handling the offerings.