A week after Harry arrives in Diagon Alley, the shop displays a prototype of “the most magnificent broom he had ever seen in his life.” It’s a Firebolt, reputed to be the fasted broom in the world. At the end of the placard giving the details of the broom are the words “Price upon request.” Harry is sorely tempted, but decides that his Nimbus 2000 would do fine. However, he returns almost every day of the next two weeks to stare at the Firebolt.
According to PA4:
"But the thing that tested Harry's resolution most appeared in his favorite shop, Quality Quidditch Supplies, a week after he'd arrived at the Leaky Cauldron."
Harry arrives in Diagon Alley on August 7, which in 1993 was a Saturday. The phrase "a week later" is vague, but we can say that the Firebolt appears around the 14th of August.