The festivals detailed below are recorded in the ancient Irish myths and of them the most important was undoubtedly Samhain.
This Irish pattern of festivals is often quoted as being typical of Celts throughout Europe but this is an assumption which needs to be treated with some caution. The literary evidence is not particularly helpful as none exists for Scotland, there is little relating to Gaul, and the Welsh sources attach no particular importance to Imbolg, Lughnasadh or Samhain,  providing details of celebrations relating only to Beltane.
The study of folk custom provides additional material and there is evidence that the importance of Samhain was recognised throughout Ireland, the Scottish Highlands and Wales and Cornwall.
Similarly evidence has been found for the widespread celebration of Lughnasadh, Beltane and Imbolc throughout the Gaelic areas of the British Isles. This would seem to suggest that the four Irish Quarter Days were celebrated in the Celtic areas of Britain at least and may have had some prevalence in European areas as well.

Samhain   
31 October
Samhain was the eve of the Celtic New year, a mysterious time when the veil between the worlds of the living and the dead was very thin. This was the time when the cattle were slaughtered as it was not possible to feed the entire herd throughout the winter months. The meat would be salted and preserved against the coming lean time. The rituals of Samhain were concerned with the dead, divination and story telling.

Imbolg
2 February
Imbolg means "in the belly" and it represents the quickening of the year and the first stirrings of Spring. This festival brings to an end the rule of the Cailleach in her guise as crone which began at Samhain and sees her transformed into a young and fruitful goddess, often identified as