Originally: a day on which a Christian martyr suffered death, or on which the martyr's sufferings are commemorated. Later: (in extended use) of non-Christian martyrs.
Origin
Early 17th century (in an earlier sense). From passion + day.
Definition of passion day in US English:
passion day
nounˈpaʃn deɪ
Christian Church rare
Originally: a day on which a Christian martyr suffered death, or on which the martyr's sufferings are commemorated. Later: (in extended use) of non-Christian martyrs.
Origin
Early 17th century (in an earlier sense). From passion + day.