I don't know who chose the date, but it's been around since the sixth century.
The Feast of the Conception of Mary was chosen about a hundred years later, by simply subtracting nine months from the Feast of the Nativity of Mary.
Only Roman Catholics call the Conception of Mary the Immaculate Conception, and they've only done that since 1854, when the Pope declared that it was infallible doctrine that she was conceived without original sin. Anglicans and Eastern Orthodox celebrate the feast but leave off the "immaculate" part.
Thank you, Google and Wikipedia.
What do you think? The extra-special Catholic Steve Bannon placed a collect call from prison to tell Trump to pretend to know this and get more RC votes?
If so, backfire! Even Catholics tend not to know about this date, so his TS message just adds to Republicans' growing impression that he's lost it. 
Everybody else already thought that, of course.
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