Words for month in Celtic languages.

Proto-Celtic	*mīns = month
Old Irish (Goídelc)	mi = month
Irish (Gaeilge)	mí [mʲiː] = month an mhí seo cáite = last month an mhí seo = this month an mhí seo chugainn = next month
Scottish Gaelic (Gàidhlig)	mìos [miəs] = month a’ mhìos sa chaidh = last month a’ mhìos seo = this month an ath-mhìos = next month
Manx (Gaelg)	mee [miː] = month yn vee shoh chaie = last month yn vee sho = this month yn vee shoh çheet = next month
Proto-Brythonic	*mis = month
Welsh (Cymraeg)	mis [miːs / miːs / miːʃ] = month, oestrus y mis diwedda(f) = last month y mis hwn = this month y mis nesa(f) = next month
Cornish (Kernewek)	mis = month
Breton (Brezhoneg)	miz = month ar miz diwezhañ = last month ar miz-mañ = this month ar miz kentañ = next month

Etymology (month): possibly from the Proto-Indo-European *mḗh₁n̥s (moon, month) [source].

Words marked with a * are reconstructions.

The names of the months (and days and seasons) in Celtic languages

Sources: Wiktionary, Am Faclair Beag, Online Manx Dictionary, Teanglann.ie, Geiriadur Prifysgol Cymru, Gerlyver Kernewek