On December 8th, the Eastern Orthodox Church celebrates the feast of the Conception of the Theotokos (a polyelei rank feast, though not numbered among the 12 Great Feasts).
On December 9th, the Roman Catholic Church celebrates the feast of the Immaculate Conception of the Blessed Virgin Mary.
These are historically the same feast (though it was elevated in the West post-schism with the formation of the doctrine of the Immaculate Conception), but they are celebrated on different days. I can find information that says the Eastern date is older, and it was later changed in the West, but I can't find information on when exactly the change occurred or why. I was wondering if anyone knew why the date was moved, especially only one day? Further, what is the practice of the Eastern Catholic Churches for this feast? All information is appreciated!