"There's a customer out here looking for a joke cauldron, Mr. Weasley and Mr. Weasley."
-- Verity, who works at Weasley's Wizard Wheezes (HBP6)
As they stand ready to leave Hogwarts, flying off into the sunset on their brooms, they announce to the awed crowd of students: “If anyone fancies buying a Portable Swamp, as demonstrated upstairs, come to number ninety-three, Diagon Alley – Weasleys’ Wizarding Wheezes. Our new premises!”
Fred and George Weasley worked for several years to develop a line of comic magic items which they hoped to sell through their company, Weasleys’ Wizard Wheezes. They were putting together order forms during the summer of 1994 with the hope of selling the items at school, but their mother was thoroughly disgusted with the whole thing and made them throw the forms away. They were undaunted, however, and they managed to impress Ludo Bagman, the head of the Department of Magical Games and Sports, with one of their fake wands. At the end of the 1994 – 1995 school year, Harry Potter gave the Weasley twins a thousand galleons he had won in the Triwizard Tournament as seed money to start them with their business.
Initially the twins ran their business as an owl-order service. Over the 1995 – 1996 school year, the twins attempted to hire beta testers among the other students (they themselves always served as the first test subjects (HBP6)), but Hermione curtailed their use of other students as testers. They were able, however, to conduct market research, conduct further product development, and build up their customer base over the course of the school year, since their focus was on developing their new business rather than studying for N.E.W.T.s.
When Umbridge finally pushed the pair too far, they decided to chuck the whole thing and flew off from Hogwarts on their brooms, since they’d already set up their shop in Diagon Alley and didn’t care a bit about N.E.W.T.s anyway.
All we know is that, since the chapter before Fred and George leave Hogwarts took place on a Monday in April, the chapter in which they leave must take place Tuesday and Wednesday of the same week. Hermione also says that the Easter holiday begins Friday, but since the calendar doesn't align to a real-world calendar, it still doesn't help determine exact dates.