I feel the need to note that the Welsh do have a word that may be the equivalent of døgn:
- diwrnod {-au, -iau, m.}: dee-OOR-nod.
It is used in the sense of 24 hours. Dydd means "day" as opposed to "night"--nos {-au, nosweithiau, f}. Nos means "night". However, noson or noswaith (that is "gwaith" as in "period", rather than "work") is a period at the end of a day, like "evening." One evening would be a "noson" or "noswaith", and there is some ambiguity between "evening" and "night".