Time
The Celtic calendar is a lunar calendar, but apparently, the months don't start at new moon like in most lunar calendars, they start at full moon. It is mentioned in ancient histories that the Celts in Spain offered sacrifices to an unknown deity on the doorstep each full moon, and in more recent times, dancing all night at full moon has been a charming tradition in Celtic countries. The old Celtic calendar found at Coligny in the south of France is not a natural calendar. In a natural calendar, observation of the new moon determines a new month and observation of the solstice determines a new year. Observatories such as the stone rings or passage graves are built to find the solstices and equinoxes. But in the Coligny calendar, which dates from the last century BCE, a mathematical rule has been invented to dispense with the old observatories, corresponding to the leap year rule of the current Gregorian calendar, by making 30 years equal to 371 months. The Coligny calendar loses almost a day and a half in those 30 years, so it is not as mathematically sound as the Gregorian. The Julian, introduced by the Romans at about the same time, gains less than 1/4 of a day in 30 years, which is better as well, but the Coligny at least is better than a pure solar calendar with 365 days each year, losing more than 7 days in 30 years.
The year starts in the fall, with the month Samonios, 'seedfall'. The leap month, Mid Samonios, is next, but only twice every 5 years and once in the last 5 years of the 30 year cycle. Then comes Dumannios, 'the deepest darkness', and then Riuros, 'cold time', Anagantios, 'inability to leave the house', and Ogronios, 'ice time'. The spring starts with Cutios, 'wind time', and continues with Giamonios, 'showing shoots', and Simivisionios, 'brightness time'. The midsummer month is Equos, 'horse time', then there are the harvesting months Elembiuos, 'claim time' and Edrinios, 'arbitration time', when products are marketed and bargained about. The last month is Cantlos, 'song time', the time of celebrating and poetizing the exploits of the year.
Just as the year begins at the fall, the day begins at sunset. This is why all feasts in countries with Celtic traditions start the night before, like Christmas eve, Halloween eve etc. Also, time is not counted as days, but nights, like in the English expression 'a fortnight ago'. The day is divided in 8 tides of 3 hours each. Their names are known in modern Welsh: Dewaint (midnight), Pylgeint (dawning), Bore (morning), Anterth (lack of vapour(?)), Nawn (noon), Echwydd (rest), Gwechwydd (dusk), and Ucher (disappearance or overcast). Today, Dewaint is reckoned from 11:30 PM until 1:30 AM and the others follow with 3 hour intervals.
There are three seasons: summer, winter and spring. The biggest festival of the year is the new year's eve, which also marks the end of the harvest and the beginning of winter. Today, it is celebrated as Hallowe'en. The second biggest is on the opposite side of the year, at the start of summer. It is known as May Day in many countries, including non-Celtic ones. The start of spring is celebrated 3 months earlier, at Candlemas or St. Bridget's Day, and the start of the harvest 3 months after, at Lammas. The four points of the Sun, the solstices and the equinoxes are also important times recognised by the Celts, and moderne Pagans often use an eightfold calendar with usually somewhat anglicized Irish names of the biggest feastdays: Samhain, Imbolc, Beltane and Lughnasa in chronological sequence. Samhain is the same as Samonios in the Coligny calendar, but the other are changed. Beltane and Lughnasa are doubtlessly connected with the gods Belenos and Lugos respectively.
The Coligny calendar is an attempt at calculating the times of the calendar feasts mathematically and doing away with astronomical observations. But traces of Celtic activity has been found associated with the megalithic monuments from the stone age as well. With a stone ring or passage grave like New Grange in Ireland, it is easy to pinpoint the summer or winter solstice, because if they are set up on a reasonably level plain with good view of the horizon in all directions, the Sun will rise at winter solstice in the exact opposite direction of the sunset at summer solstice, and at the equinoxes, the Sun rises in the exact opposite direction of the sunset. Next time you see a stone ring or passage grave, check if the two tallest stones in the ring or the passage in the passage grave are set in the northwest-southeast direction to line up the Sun at the two solstices. Sometimes you may also find stones or passages set up exactly east-west to find the equinoxes.
The mathematical nature of the Celtic calendar shows that the Celts at Coligny around the last century BCE did not use the megalithic monuments for time measurement, at least they were not dependant on them. Possibly, they used them for supplementary solstice observations, or only as backdrop for their ceremonies, for which they are of course admirably suitable. However, noone can tell how the Celts measured time 2-3 or more cenuries before or at a different location.