Who says it’s neglected or lost? It’s all there in Luke - it mentions Zechariah offering incense in the Temple when John’s conception was announced. Going in on his own, remember, they thought something had happened when he took so long?
If I remember rightly, there was only one occasion during the year when this happened (priest offering incense alone), which was the Jewish New Year.
This year JNY was mid-September (although JNY is one of those irregular dates like Easter, I believe, being calculated by a lunar rather than solar calendar). That seemingly puts John’s conception around that date. Gives us roughly mid-June for John’s birth and mid-December for Christ’s birth, which isn’t too far off
Then Luke makes it very clear by saying that Gabriel appeared to Mary ‘in the sixth month’. Meaning in fact not the sixth month of Elizabeth’s pregnancy (although it was that as well), but the sixth month of the Jewish year too So it confirms that same rough date for John’s and Jesus’ conceptions and births.
It’s in no way central to the Pascal Mystery and it’s not really relevant that we know on what day or month Jesus was born.
The liturgical calendar celebrates all of the Feasts relevant to our Salvation, and it does so in a compressed manner that makes sense liturgically, not chronologically. And with Easter as the central Feast on the calendar.
why is it critical? why does this matter so much to you and others who have started the dozen other threads on the topic? Divine Revelation preserves and hands on through scripture and tradition all that is necessary for our salvation. If the traditional date of celebrating the birth of Christ, under either the old or new calendars, is not good enough for you, the burden of proof is on you to claim this tidbit is necessary for our salvation.
You are missing the point. The date of Christmas was reassigned to Dec 25th, to disguise it within the pagan festival of the longest day of the year. Why doesn’t the church return it to it rightful day or least month, which by biblical indications was in the Spring?