Officers are looking for anyone who saw a man attacked outside a pub in Hebden Bridge.
The victim had tried to calm an argument between two women as he was walking into the Hole in the Wall when he was assaulted by another man.
He suffered head and face injuries.
The attacker is described as white, around 20 years old, around 6ft 1ins tall and of medium build.
He has a bold jaw-line and was wearing a white long-sleeve top and jeans.
Anyone with information about the attack, at 11pm on Friday, December 16, should call PC Nichola Todd at Calderdale Police on 01422 337169.