Two things to note while answering your question.
Harry's family died in the PM on Halloween, Harry does not arrive at the Dursleys until the PM of the following day, November 1st.
We only have clear celebration starting the morning of November 1st, Mr Dursley notices it on his way to work.
Taking these things into account, the Daily Prophet was most likely to culprit of the wide spread knowledge of at least Voldemort's disappearance, if not outright stating he was gone.
But to further speed up the spread of information, the Prophet is also known to release an evening issue.
as Snape unrolled today’s issue of the Evening Prophet.
“You were seen,” he hissed, showing them the headline: FLY-
ING FORD ANGLIA MYSTIFIES MUGGLES. He began to read
We know Voldemort's attack came while Harry was still awake for the night, and he lived in a Wizarding community so other wizards would have been aware of the house being blown up. Not hard to imagine after that a quick owl to the Prophet, and an evening/late night special report to get sent out asap.
Note again, the celebration is still semi speculative, as Professor McGonagall states.
‘What they’re saying,’ she pressed on, ‘is that last night Voldemort
turned up in Godric’s Hollow. He went to find the Potters. The
rumour is that Lily and James Potter are – are – that they’re –
Beyond that, the scar on Harrys forehead, and the house blowing up, and lack of a dark mark in the sky all shed light on something amiss from the usual order of business. Even by the time Harry is 11 Hagrid says speculation still abounds.
‘But what happened to Vol– sorry – I mean, You-Know-Who?’
‘Good question, Harry. Disappeared. Vanished. Same night he tried ter
kill you. Makes yeh even more famous. That’s the biggest myst’ry, see
... he was gettin’ more an’ more powerful – why’d he go? ‘Some say he
died. Codswallop, in my opinion. Dunno if he had enough human left in
him to die. Some say he’s still out there, bidin’ his time, like, but
I don’ believe it. People who was on his side came back ter ours. Some
of ’em came outta kinda trances. Don’ reckon they could’ve done if he
was comin’ back.