For defensive purposes (and Celts were warriors first and foremost) each ‘day’ had eight defensive watches of three hours each.
Midnight 22.30 – 01.30
Dawn 01.30 – 04.30
Morning 04.30 – 07.30
Mist-rise 07.30 – 10.30
Midday 10.30 – 13.30
Rest 13.30 – 16.30
Dusk 16.30 – 19.30
Disappearance 19.30 – 22.30
The most evocative watch is ‘mist rise’ which, in the language of the Gaulish Celts was ‘anthert’ or absence of vapour (an=absence of, thert [presumably] vapour or mist). I can well imagine the Celts pottering around until well into our morning waiting for the overnight mist to clear so they could go about their business.
The most intriguing is ‘rest’ (13.30-16.30) which seems both a long and ill-timed place to have a few hours off. I don’t think the times are indicative of a three-hour-long siesta (a tradition from Southern Europe) but rather the watch came to be named after a shorter period of rest that happened in the middle of the working day. Our phrase ‘noon’ which we apply to ‘midday’ comes from the Old English non which means ‘the ninth hour from (6am) sunrise’. Or 3pm. Smack in the middle of the (older) Celtic ‘rest’. Given the possible late start to the day waiting for the mist to clear, I think it’s possible that Celts worked through until stopping for a meal and a break around our 3pm and then perhaps resumed again until the light started to run out toward the end of ‘dusk’.
The passage of broader time across months and years were tracked by the movement of star constellations across the heavens and by the moon respectively. Therefore, Celts are believed to have had a lunar/solar calendar with a very straightforward (but effective) way of countering for seasonal ‘drift’. Check it out in these posts about the Coligny Calendar, the Celtic Week, Celtic Month and Celtic Year.