In the first aliyah (section) of Acharei Mos it speaks about the service of the Kohen Gadol (high priest) on Yom Kippur. This process is the ritual for coming into the Kodesh Kadashim, the holy of holies, and coming close to Hashem. Aharon can enter whenever he wished, but for generations, it will be limited to only on Yom Kippur. The core of atonement which is what we try to attain on Yom Kippur, is the encounter of the person with Hashem as close as can be. We need to be holy and practice the Mitzvos, some of which are listed in the first aliyah of Kedoshim, in order to merit this close encounter.