Immaculate Conception
By 750, the feast of her conception was widely celebrated in the Byzantine East, under the name of the Conception (active) of Saint Anne. In the West it was known as the feast of the Conception (passive) of Mary, and was associated particularly with the Normans, whether these introduced it directly from the East or took it from English usage. The spread of the feast, by now with the adjective "Immaculate" attached to its title, met opposition on the part of some, on the grounds that sanctification was possible only after conception. Critics included Saints Bernard of Clairvaux, Albertus Magnus and Thomas Aquinas. Other theologians defended the expression "Immaculate Conception", pointing out that sanctification could be conferred at the first moment of conception in view of the foreseen merits of Christ, a view held especially by Franciscans.

Q: What was widely observed in the eastern portion of the Byzantine world ?
the feast of her conception was widely celebrated in the Byzantine East