tan ddydd Sul y pys

== Welsh == === Etymology === Literally, “until the peas' Sunday”. === Phrase === tan ddydd Sul y pys (idiomatic) when pigs fly, when two Sundays come together, never ==== Related terms ==== bwyta uwd â mynawyd (“to herd cats”, literally “to eat porridge with an awl”) 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”) pan fydd yr Wyddfa’n gaws (“when pigs fly”, literally “when Snowdon is made of cheese”) unwaith yn y pedwar amser (“once in a blue moon”, literally “once in the four seasons”) unwaith yn y pedwar gwynt (“once in a blue moon”, literally “once in the four winds”)