The article by Daibhi O Croinin on 'The earliest Irish names for the Days
of the Week', Eriu, 1981, should have all the information and references
for the early Celtic calculation of time, he's also gots a number of
papers on the Irish calculation of Easter which might be of some interest
to you, most of these can be found in the journal Peritia.
An Iron Age Calendar from Gaul might also be of interest to you, its
called the Coligny Calendar, should be in a lot of reference works.
Mark Handley, Trinity Hall, Cambridge.