All Hallows Eve = Samhain
Christmas = Yule
Candlemas = Imbolc
Easter = Ostara
Litha = Whit Sunday
Lughnasada/Lammas = St Mary's Mass
Mabon = Harvest Festival
The adoption of pagan holidays and conversion to Christian meanings started with the Roman Church (the predecessor to the the Catholic Church) and was done for reasons of political expediency.
The Roman Emperor Constantine needed a religion that would unite his failing empire, so paganism and Christianity were merged.