The Lord wanted the high priest Eleazar to witness the slaughter of a red heifer outside the camp. The red heifer could not have any blemishes, or defects, and it could never have borne the yoke.
When the red heifer had been slaughtered, the priest then took some of the blood and went back to the tabernacle,
“and Eleazar the priest shall take some of its blood with his finger, and sprinkle some of its blood seven times directly in front of the tabernacle of meeting” (Numbers 19.4).
The Lord wanted that blood sprinkled seven times, for that would perfect and complete the cleansing of the tabernacle.
Even as seven days make a complete week, so sprinkling the blood seven times before the tabernacle cleansed and purified it perfectly, and completely.