The Gregorian calendar (‘Western’ calendar or ‘Christian’ calendar) that most of us know so well is strictly a solar calendar — meaning it generally disregards what the moon is doing and focusses more on the patterns of the sun (which the Earth fully rotates around in 365 days and nearly six hours). That six hours causes a 24 day outage over a century. The Gregorian calendar copes with that by having irregular (28, 30 or 31) which don’t neatly align with the moon, a short 28 day month assigned to February and a ‘leap day’ (Feb 29) assigned once every four years to drag things back inline with the sun across a century.
Celtic timekeeping on the other hand (based on knowledge gleaned from the Coligny calendar unearthed in Gaul) was luni-solar which meant the Celts observed the patterns of the moon for (12) months and the sun for their years and ‘ages’ (and ‘age’ was half of our ‘decade’). The two crept out of synch after a while and so the Celts managed it with the neat inclusion of leap (interclary) months every 2.5 ‘years’. The Celtic calendar looks a lot like the ancient Hebrew calendar structurally and they share very similar patterns of 29 and 30 day months as well as the habit of adding the (interclary) month now and again to keep the solar calendar aligned. But whereas the dualistic Celts simply added their interclary months at the start and middle of their 60 month ‘age’ cycle (dividing each age into two halves) the Hebrews applied the more random habit of adding 7 interclary months across a 19 year period. Probably more mathematically accurate but messy and never the same twice.
Like the Hebrews (who considered 30 day months as ‘full’ and 29 day months as ‘deficient’), or the Greeks (30 = ‘full’ and 29 = ‘hollow’), or the Babylonians (30 = full’ 29 = ‘defective’), the Celts referred to the 30 day lunations as ‘matos’ (lucky) and those of 29 days as ‘unlucky’ (anmatos).
Either way the poor old 29 day month is generally maligned.