Where does “blwyddyn” come from?

blwyddyn (Middle Welsh) comes from Old Welsh bloidin, from Proto-Celtic blēdanī, from Proto-Indo-European bʰloyd- — pale.

Ancestry of “blwyddyn”, step by step

Step	Language	Word	Meaning
1	Old Welsh	bloidin	—
2	Proto-Celtic	blēdanī	year
3	Proto-Indo-European	bʰloyd-	pale

blwyddyn — Etymology, Origin & Word History | Etymology Explorer