If you can understand that Dec 25 is not likely Jesus’ birthday, but is the day we celebrate His birth, then you can understand that a day to celebrate His Incarnation in the womb is not the same as saying “this is infallibly the day Mary became pregnant.”
I am not suggesting a new holiday. It’s already on the church calendar.
Oh heck!
Let’s just figger out John the Baptist’s birthday and work from there!