Refers to a gathering held the night before a funeral, similar to a wake in Christian contexts. The term is not widely used in everyday Japanese; 通夜 (つや) is the more common word for a Buddhist wake.
A 前夜式 is a gathering held the night before the funeral.
Compound of 前夜 (ぜんや, 'the previous night') and 式 (しき, 'ceremony'). The exact origin is uncertain, but it likely arose as a translation of the Christian concept of a wake.