It was only available during the 2nd Anniversary event. HOWEVER, it has been hinted that it may be available at other times as well. Probably when each new season is released or at each anniversary event. Also, some interesting details.
I use a tier 4 Assault Cruiser. yet my VA buddies used Odysseys. I was the only one of us not to explode during my first STF. The Odyssey is not as good as everyone believes. It's Boff stations are not up to par in terms of layout, it has a slow turn rate, and is more for show than combat. It's basically to say "I played during the event, and am a VA" Stick with your standard Tier 5 VA ships. Only try to obtain an Odyssey AFTER you have bought ship slots and feel like wasting one.
Like any ship the Odyssey has its ups and downs, the turn rate does suck but a few RCS consoles fix that plus if you have the Operations version dropping the saucer really improves turn rate. It's all abt preference really, sorry you missed the free one though! The c-store ones are better IMO and you get a new bridge not some dumb redress they did for the 1st Odyssey.