Final answer:
Fred and George's impromptu joke shop was set up on the seventh floor of Hogwarts Castle before they opened Weasleys' Wizard Wheezes in Diagon Alley.
Step-by-step explanation:
Fred and George Weasley set up their impromptu joke shop on the seventh floor of Hogwarts Castle. This is before the entrepreneurial twins opened their successful business, Weasleys' Wizard Wheezes, in Diagon Alley. The joke shop at Hogwarts was an early indication of their ambition and penchant for mischief, which later translated into a thriving enterprise that provided a wide range of magical novelties and practical joke items to the wizarding community.