Hmm, a later festival held in mid-August was celebrated by Celtic Christians as early on as AD 900. This harvest festival was pragmatically located later in the month than Lughnasadh for the cool Northern climate. It sez all about Marymass 'ere.:

"Very early on, the Church chose August 15th to honor the Assumption of the Blessed Virgin Mary. August 15 would be the full moon of August if the new moon fell on the first of the month as it did during the time of the lunar calendar. It was proclaimed a holiday throughout the Roman Empire by Emperor Maurice around 600 in the East, and about 50 years later in the West. Common Celtic people would not have been aware of the theological doctrine of the assumption—that Mary did not die but was taken bodily up into heaven at the end of her earthly life. However, Celts would have associated Mary with the fruitfulness of the earth at the time of harvest, and celebrations connected to other harvest goddesses were transferred to Mary as the pagan Celtic lands were converted to Christianity. Representations of Mary often resembled the ancient depictions of harvest goddesses wearing robes decorated with ears of corn.

Marymass was the primary harvest festival in the northern Celtic regions where the harvest was later. It replaced the southern harvest festival Lughnasa which was normally celebrated anytime from late July to early August. The offering of the first fruits, first grains, or first loaf of bread, or Lammas (loaf-mass) which usually occurred during Lughnasa in the south, was transferred to Marymass in the north. The Lammas bannock (a traditional Scottish loaf) would be made from the new corn, dedicated to Mary the Mother of God, and used in Eucharistic celebrations. Some other traditional Marymass activities, such as elaborate flower displays, well dressing, torchlight and candlelight processions, and the offering of the first bread, were preserved intact for centuries because of the isolation of the Scottish islands and highlands."