West Yorkshire Police today released details of the frightening incident as they urged any witnesses to come forward.
The robbery happened in Lumley Road, Dewsbury at atound 4.30pm on Tuesday when the occupants answered a knock on the door.
The elderly man opened the door after hearing carol singing, only to be confronted by a man with a knife.
He fell to the floor and suffered minor injuries, before the robber demanded cash and money was handed over to him.
Detective Inspector Stephanie Fox, of Kirklees CID, said: “This was a particularly nasty robbery against two elderly people. In the end the male only made off with a small amount of money.
“The victims were very frightened by the ordeal and I would appeal to any witnesses to the incident or who saw anyone acting suspiciously in the area at around 4.30pm to come forward."
The robber was described as a white man, around 5ft 10 and of a stocky, muscly build.
He appeared to be over 40 years old and had an "all over number one" shaved head.
He wore a plain grey anorak with a hood, a dark coloured scarf covering his mouth and nose, and grey tracksuit bottoms.
Det Insp Fox added: “We would always urge people to be extra vigilant when answering the door to unexpected callers – for example, by having a chain on the door and using it."
Anyone with information is urged to call 101, quoting crime reference 13180633079.