On what exact date was Windows 1.0 released to retail?

Windows 1.0 was released to retail on November 20, 1985.

More precisely, the first retail package was Windows 1.01, released in the United States on that date for US$99. Microsoft presented Windows as a graphical operating environment that ran on top of MS-DOS, rather than as a completely independent operating system. It provided a mouse-driven interface and a collection of early graphical applications.

Windows 1.0 included Paint, Notepad, Calculator, Write, a clock, a control panel, a terminal program, and an appointment calendar. Its windows could not freely overlap; they were arranged in a tiled layout. The software also required modest hardware by later standards, including an 8088 processor and at least 256 KB of memory.

The product had a long development period. Microsoft publicly demonstrated Windows in 1983, but the retail launch came roughly two years later. Regional and hardware-focused revisions followed: Windows 1.02 appeared in Europe in May 1986, Windows 1.03 followed in August, and Windows 1.04 arrived in 1987 with IBM PS/2 support. Windows 2.0 succeeded it in December 1987.