William Day
William, Will, Bill, or Billy Day may refer to:
Arts and entertainment
- William Day (lithographer), British lithographer and watercolour artist
- Bill Day (filmmaker), American documentary filmmaker
- Bill Day (cartoonist), American cartoonist
Law and politics
- William Day (Australian politician), Australian politician in New South Wales
- William R. Day, American diplomat and Supreme Court Justice
- William Louis Day, American judge
- William J. Day, American judge from Massachusetts
- William S. Day, American politician in the state of Washington
Sports
- William Day (horseman), British racehorse trainer
- Bill Day (baseball), American baseball player
- Bill Day (alpine skier), Australian Olympic skier
- Billy Day, English footballer
- Will Day, Australian rules footballer
Others
- William Day (bishop), English clergyman
- William Day (divine), English clergyman
- William Day (sea captain), American naval captain
- William Day (architect), Irish architect
- William H. Day, American abolitionist