William, Will, Bill, or Billy Day may refer to:
Arts and entertainment William Day (lithographer) (1797–1845), British lithographer and watercolour artist Bill Day (filmmaker) (born 1959), American documentary filmmaker Bill Day (cartoonist), American cartoonist
Law and politics William Day (Australian politician) ( 1861), Australian politician in New South Wales William R. Day (1849–1923), American diplomat and Supreme Court Justice William Louis Day (1876–1936), American judge William J. Day (1876–1950), American judge from Massachusetts William S. Day (1923–1984), American politician in the state of Washington
Sports William Day (horseman) (1823–1908), British racehorse trainer Bill Day (baseball) (1867–1923), American baseball player Bill Day (alpine skier) (1934–2024), Australian Olympic skier Billy Day (1936–2018), English footballer Will Day (born 2001), Australian rules footballer
Other people William Day (bishop) (1529–1596), English clergyman William Day (divine) (died 1684), English clergyman William Day (sea captain) (1715–1797), American naval captain William Day (architect) (fl. 1800), Irish architect William H. Day (1825–1900), American abolitionist William Scott Day (1951–2006), American prison escapee and spree killer
Other uses SS William R. Day, a Liberty ship