His name was Sandy. He’d introduced himself to a couple of Mexicans when he first moved to Mexico.
One of the Mexicans had encountered a little bit of English somewhere in his life. And to his ear, Sandy sounded quite a bit like Sunday. In Spanish, the word for Sunday is Domingo.
Sandy had encountered a little bit of Spanish somewhere in his life and tried to use it. The Mexicans liked that he tried speaking their language and tried to be inclusive with him.
So the Mexicans that Sandy met always called him Domingo.