"The Ministry of Magic is undertaking a survey of so-called 'Muggle-borns,' the better to understand how they came to possess magical secrets. Recent research undertaken by the Department of Mysteries reveals that magic can only be passed from person to person when Wizards reproduce. Where no proven Wizarding ancestry exists, therefore, the so-called Muggle-born is likely to have obtained magical power by theft or force. The Ministry is determined to root out such usurpers of magical power, and to this end has issued an invitation to every so-called Muggle-born to present themselves for interview by the newly appointed Muggle-born Registration Commission."
-- Daily Prophet, August 3, 1997 (DH11)
With the fall of the Ministry to the Death Eaters on August 1, 1997 (DH9), the attitude of the bureaucracy changed immediately toward the persecution of Muggle-born witches and wizards. A leader in this effort was Dolores Umbridge, who headed up the Muggle-Born Registration Commission which was tasked with imprisoning Muggle-borns on the charge of having “stolen” their magic (DH12).
The Ministry fell on Thursday, August 1, and Lupin showed Harry the Daily Prophet article about the Ministry investigating Muggle-borns on Sunday, August 4. The change in attitude at the Ministry was indeed swift. Since the Ministry likely wouldn't have been open on Saturday, it is likely that the investigation of Muggle-borns started up already on Friday, August 2.