Dr. Anne Ross has written some books which you may find useful.  For the historical/archaeological perspective, look at Pagan Celtic Britain (Routledge & Kegan Paul,1967 /Cardinal paperback,1974) and Everyday Life of the Pagan Celts (Batsford/Putnam, 1970); for more recent material -including sections on seasonal customs- there's Folklore of the Scottish Highlands (Batsford 1976; paperback 1990).

Malcolm