A group of three turian officers can be found chatting inside Purgatory, in an alcove to the right of where James is located at the bar. Listen to their conversation to acquire the mission. You can also acquire it if you scan the planet first. This second way of acquisition is possible even before the Purgatory becomes available on the Citadel. The name of the mission in the journal will then change to Citadel: Banner of the First Regiment.
Scan Digeris, the third planet in the Castellus system of the Apien Crest cluster. The Banner of the First Regiment is located there. Return to the Citadel and speak to the turian officer to finish the mission.