We don't know all the festivals they celebrated, but they had four important ones during different seasons. After the Romans took over Gaul and southern Britain, the Celtic religion mixed with Roman ideas. Later, the Celts started following Christianity, and their old religion faded. However, Celtic mythology and beliefs had a lasting influence and inspired a modern religious movement called Celtic neopaganism. Therefore, all there is to study their religion from is the literature from the early Christian period, commentaries from classical Greek and Roman scholars, and archaeological evidence.