Experts are divided on what phase of the moon the Celts started their month. Modern, western society starts ours on the ‘new’ moon (when it’s fully dark). Some think the Celts started on the full moon because that’s when they celebrated festivals etc.
But Pliny the Elder (who, it must be said, captured many correct details of popular culture of his time but managed to either invent or misreport many others) wrote of the Celts beginning their month on ‘the fifth day of the moon’. This seems like a bit of a random time to start a month until you consider that it coincides with the quarter-moon, a phase in which the moon appears exactly half/half light and dark to us on Earth.
Half/half is a pretty unequivocal kind of shape (compared to the three-day linger of a new and full moon) because every eye turned to the sky could see when the moon was exactly divided (even one day before or after a quarter-moon is visibly not half/half). And it fits with the common Celtic practice of interpreting their world dualistically.
Starting the Celtic month at a quarter-moon ensures that the first fortnight is ‘light’ as the moon gets fuller and brighter, and that the fortnight following it is ‘dark’ as the moon goes back to being in shadow. In fact, the Celts seemed to call them a bright moon and dark moon respectively (rather than ‘full’ and ‘new’ which are contemporary terms). This fits with references in the Coligny Calendar to the Mat and Anmat periods in the first and second halves of the month. (mat meaning ‘light’, and an as we’ve seen elsewhere meaning absence of).
Therefore, if a month began on the quarter moon (half/half) then the mat fortnight would take us through the brightest part of the month, until the three-quarter moon (where the half/half is reversed) and then the period marked as Anmat would predominantly feature a dark moon. But… the Coligny Calendar clearly marks the second fortnight of each month with the Gaulish word Atenoux which has been translated as the return of light. The dark fortnight would most definitely end in the return of light. So perhaps the Atenoux fortnight was all about looking forward to the moon’s return.