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The Celtic language with the most speakers as their mother tongue is Welsh so I will assume you are asking to say it in Welsh - "Nos da". In the other Celtic languages it is "…Noz vat" (Breton), "Oíche mhaith" (Irish Gaelic), "Oidhche mhath" (Scottish Gaelic), "Nos dha" (Cornish) and "Oie vie" (Manx).
Perhaps this information on "Answers.com" may assist you in learning and forming your own deduction: http://www.answers.com/topic/pronunciation-of-celtic
Irish: abhainn Scottish: abhainn Welsh: afon Breton:
In Irish it's: madra / gadhar In Welsh it's "ci" In Scots Gaelic it's "cù"
In Irish it's "aonbheannach" In Welsh it's "uncorn" In Scots Gaelic it's "aon-adharcach"
In Irish "ridire" In Welsh "marchog"
Irish: toirneach Scots Gaelic: tàirneach; tàirneach; torrann Welsh: taran(au), tyrfau, trystau
In Irish it's "sliabh" In Scots Gaelic it's "beinn" In Manx: In Welsh it's "mynydd" In Breton: In Cornish:
cairde (Irish) càirdean (Scottish Gaelic) cyfeillion, ffrindiau (Welsh) karidi, keveiled (Breton)
There's actually no such language as "Celtic". Celtic refers to a group of dozens of languages, six of which are spoken today: BretonCornishIrish GaelicManxScottish GaelicWel…sh
Irish Gaelic: cloch Scottish Gaelic: clach Welsh: carreg or maen
Irish: noun: drithle, spréacharnach; verb: drithligh, glinnigh Scottish Gaelic: noun: splang, lainnir, sradrach verb: srad, loinnrich Welsh: verb: gwreichioni, serennu, pef…rio
Irish Gaelic: mam, daid Scots Gaelic: mamaidh, dadaidh Welsh: mam, tad/tada/tyta/dada