This is how Aaron shall come into the sanctuary… (Leviticus 16:3)
After being told that the High Priest shall not enter the Holy of Holies indiscriminately, we are told the context in which he should enter: to perform the Yom Kippur service. This mitzvah encompasses the entirety of the Yom Kippur service, which must be carried out in the order prescribed.
The rest of the chapter describes the order of the service: the clothes worn by the High Priest when he officiates, the lots drawn over two goats, the personal chatas (sin offering) of the Kohein Gadol, the incense service, sprinkling the blood, the sending away of one goat to a rocky place, the burnt offerings and more. (All told, there were fifteen animal sacrifices including the two regular daily sacrifices that were normally offered.)
All this service was performed personally by the Kohein Gadol. He immersed in a mikvah a total of five times and would change clothes between his white uniform and his gold uniform. He also washed his hands and feet a total of ten times.
The reason for this mitzvah is that God is really good to us! He set aside one day a year on which He is extra-receptive to our teshuva (repentance) and gladly welcomes us when we return to Him. On Yom Kippur, God doesn’t just forgive, He actually wipes the slate clean. But there’s a catch. You know the saying, “God helps those who help themselves?” Well, He won’t do all this FOR us so much as He’s willing to do this WITH us. We can’t sit around expecting to be cleansed automatically; we have to do our part. In Temple times, this meant following the order of the sacrificial Yom Kippur service. Nowadays, it means the Yom Kippur prayer service in our synagogues (as well as fasting and actually repenting our sins!).
This mitzvah was observed in Temple times. It is the subject of the Talmudic tractate of Yoma and is codified in the Mishneh Torah in Hilchos Avodas Yom HaKippurim. It is #49 of the 248 positive mitzvos in the Rambam’s Sefer HaMitzvos.