unwaith yn y pedwar amser

Literally, “once in the four seasons”.

unwaith yn y pedwar amser

1. (idiomatic) once in a blue moon, very rarely

• bwyta uwd â mynawyd (“to herd cats”, literally “to eat porridge with an awl”)
• tan ddydd Sul y pys (“when pigs fly”, literally “until the peas' Sunday”)
• pan fydd yr Wyddfa’n gaws (“when pigs fly”, literally “when Snowdon is made of cheese”)
• pan fydd Nadolig yn yr haf a gwsberis yn y gaeaf (literally “when Christmas will be in the summer and goosberries in winter”)
• pan fydd uffern yn rhewi drosodd (“when Hell freezes over”)
• unwaith yn y pedwar gwynt (“once in a blue moon”, literally “once in the four winds”)

• D. G. Lewis, N. Lewis, editors (2005–present), “unwaith”, in Gweiadur: the Welsh–English Dictionary, Gwerin
• R. J. Thomas, G. A. Bevan, P. J. Donovan, A. Hawke et al., editors (1950–present), “unwaith yn y pedwar amser”, in Geiriadur Prifysgol Cymru Online (in Welsh), University of Wales Centre for Advanced Welsh & Celtic Studies