Story Arc: Invasion Day and Grand Warlord Voss
From other speculation, it appears that the Invasion Day/Freedom Five Grand Warlord Voss storyline ends with the banishing of Grand Warlord Voss into the mists by Nightmist. Accordingly, this would also be a single moment in time, although, as the Freedom Five comic shows us, a long moment. This is not a terribly helpful fact, if true, however, as Grand Warlord Voss and his cronies only appear in three comics. This is with the exception of Vyktor, who later appears, apparently on his own, in the Vengeance arc. The comics are:
Freedom Five #504, 505, 507, 509, 510, 512, 517, 536
There are no card references to 504 and 505, which are the first appearance of Grand Warlord Voss and one of the covers from the Villain Challenge Mode pdf, respectively. The remaining issues have the following references: 507 - Forced Deployment; 509 - First Lieutenant Tamar; 510 - TCF Conqueror; 512 - First Lieutenant Vyktor and TCF Stalwart; 517 - Gene-Bound Shock Infantry; 536 - Unload.
Stranger in a Strange World #8, 9
References are to Divine Focus for issue 8, and Grievous Hail Storm and Localized Hurricane for issue 9. Stranger in a Strange World is likely the story of Tempest first coming to earth, as issue #1 is Tempest's first appearance, which would mean that Tempest came to Earth only shortly before (or concurrent with) the invasion by Grand Warlord Voss.
Justice Comics #602
Reference here is Superhuman Durability. This does not appear to tie in with any particular moment within the arc, but must be before Nightmist #51, below.