Soft Day
A grey, drizzly, mild day. Not actively raining but not dry either. The default weather of much of the Irish year, named with a hint of resignation and a hint of affection. Often said in greeting: 'a soft day, thank God.'
Etymology
A calque from Irish 'lá bog' (lá = day; bog = soft). The Irish phrase uses 'soft' metaphorically to describe weather that is neither bright nor properly wet - the persistent low cloud, the fine mizzle, the air that wets clothes without ever feeling like rain. Hiberno-English borrowed the construction whole, keeping the metaphor intact. Wikipedia's Hiberno-English vocabulary section lists the phrase explicitly with its Irish source.
In a sentence
"A soft day, thank God. Are you coming in for a cup of tea?"
Historical notes
Soft day comes with a small ceremonial register: it is often said aloud as a greeting, with the optional addition 'thank God' or 'thanks be to God'. The thanksgiving is not ironic; in farming country, a soft day was preferred to a harsh wet one or a baking hot one, because it kept the grass growing and the cattle settled. The phrase has now travelled out of agricultural use into general Irish English and reads as a small piece of cultural code-switching when used.