"You will conceive and give birth to a son" (Luke 1:31).
To be honest, the date's probably out by 3 months. It's more likely that Christ was conceived on a date around December 25th and born at the feast of tabernacles in September (cf John 1:14), but let's go with the church calendar... March 25th is 9 months before "Christmas" and so today we remember the conception of Christ.
Now, think about it. The beginning of Christ's life as man (and for man) was conception. That's a vital christological truth. If you can't affirm it, you will fall into all sorts of errors. You see, there isn't an independent humanity to Christ. It's not as though there might have been a Jesus of Nazareth who wasn't chosen to be the vehicle for the Son's incarnation. The Word did not look upon a pre-existing bunch of cells and say "That'll do, I'll jump in."
No, "the Word became flesh" - He didn't adopt some flesh that looked promising. And the point at which He became flesh must be conception. You can't have the Word slipping into a pre-existing flesh without altering the gospel. All orthodox Christology demands it and Luke, the doctor, confirms. At conception God the Son became a man. To deny this is to embrace all sorts of heresies condemned down through the years (adoptionism or apollinarianism for instance).
With all that said, the feast of the annunciation ought to be the day we value the unborn more than any other. Yet today we are reeling from the revelation that thousands of aborted and miscarried babies were incinerated as clinical waste, with some even used to heat hospitals.
The world recognises that this is wrong. When confronted, the Department of Health immediately put a ban on the practice. But then why is it wrong to dispose of these remains as "clinical waste"? In cases where parents have chosen to rid themselves of their little ones, "clinical waste" is exactly how such lives have been treated. Yet over the last 24 hours there has been an outcry about such practices. Well, rightly so. But let's think about why it's wrong.
Surely it's wrong because we recognise the humanity of the unborn. They are not remains to be incinerated - or, worse, fuel for our central heating! They deserve respect in death. But if they deserve respect in death, then surely - please! - they deserve protection in life? Our hearts cry for it and Christmas demands it.