Actually what happended to me was wierder.
I saved the game after a quiet month, then fast forwarded a further 5 months, - no aliens. So I posted this thread.
Then I went back into the game and reloaded from before my 5 month fastforward and immediately go the final "ToDo" mission.
I completed this mission, got nothing else, came out of the game, posted on this thread again.
Then I went back into the game, ran the game forward and at the end of the month got the final credit screen.
I suspect some of of bug prevents the final mission and credits from coming up and the the bug is reset when the game is reloaded (or the program started up).
I suggest you try a few fast forward save/reloads to see if you can get the final credit screen, help give you some closure
Cheers.