In the first aliyah the story continues from where Aharon's sons just died by doing a service in the Mishkan without authorization. G-d here warns Aharon not to approach the ark at the wrong times, and tells him specifically to only approach it on Yom Kippur. This aliyah gives further details of the Yom Kippur service regarding Aharon's changes of clothes and all the times he immerses in the mikvah between each change of clothes, and the animals used in the sin offerings.