Just gotta a little more info on this. The Citadel DLC opens up after the Priority Citadel 2 mission.
If I recall, that's Udina's Coup attempt?
But they suggest going all the way up to the final assault mission at Cerberus HQ so you can meet all your old squadmates THEN starting the DLC so you get the full effect of said DLC.
I guess I worded that right, you'll figure it out.