Michael or Mike Day may refer to:

• Michael Day (guitarist), American guitarist
• Michael Day (cricketer) (born 1974), Australian cricketer
• Mike Day (darts player) (born 1955), New Zealand darts player
• Mike Day (cyclist) (born 1984), American bicycle motocross (BMX) racer
• Michael S. Day, member of the Massachusetts House of Representatives
• Michael Day (paleoanthropologist), British anatomist and paleoanthropologist
• Mikey Day (born 1980), American comedian