The UIT supergroup presents an instanced Rikti mothership raid every other Saturday, at 3:15 P.M. ET/12:15 P.M. PT/9:15 P.M. CET. You must be level 35+ and have the Member of Vanguard badge to join. There will be a 200 million INF kill shot prize on the Rikti drop ship!
I, Apparition, will announce in the LFG channel when the raid starts forming. Please stay where you are, do not come to the Rikti War Zone, and just send me a tell for an invite. You will then automagically be whisked away to the instanced Rikti mothership raid from wherever you are on the shard, as long as you are not in a mission or supergroup base.
For more information on the instanced Rikti mothership raid itself, please see the wiki entry here.