If Day was a simulated Nazi German invasion and occupation of the Canadian city of Winnipeg, Manitoba, and surrounding areas on 19 February 1942, during the Second World War. It was organized as a war bond promotion by the Greater Winnipeg Victory Loan organization, which was led by prominent Winnipeg businessman J. D. Perrin. The event was the largest military exercise in Winnipeg to that point.

Winnipeg is the capital and largest city of the province of Manitoba in Canada. It is centred on the confluence of the Red and Assiniboine rivers, near the longitudinal centre of North America. As of 2021, Winnipeg had a city population of 749,607 and a metropolitan population of 834,678, making it Canada's sixth-largest city and eighth-largest metropolitan area.