Where have all the young ones gone, long time ago…

"He returned on June 26, Saint John's and Saint Paul's Day, early in the morning at seven o'clock (others say it was at noon), now dressed in a hunter's costume, with a dreadful look on his face and wearing a strange red hat. He sounded his fife in the streets, but this time it wasn't rats and mice that came to him, but rather children: a great number of boys and girls from their fourth year on,"

can be read in The Children of Hameln, a folk tale as recorded by Jacob and Wilhelm Grimm (1816, Deutsche Sagen).
"In total," recorded the Grimms, "130 were lost."
There is no evidence that the legend of the Rat-catcher (or the Pied Piper) is actually based on any real event. The fact that the event is told to have happened on a specific date rather than once upon a time seems to lend the tale authenticity in the public eye. What is essentially a folk tale has been repeatedly recorded as an actual event. Being that these recordings are themselves centuries old, they lend yet another aura of authenticity to the legend.

"In the year 1284 after the birth of Christ
From Hameln were led away
One hundred thirty children, born at this place,"

is inscribed, centuries long already, on the town hall of Hameln. The town records begin with "the event" – an event that may very well have never happened. The “fact” is kept eerily alive by town regulations that prohibit any music to be heard on the Pied Piper’s (so-called “drumless”) street.
Many explanations have been offered of the actual event in 1284 that might have spawned the legend. Please refer to Wikipedia for these. As for me, I am far more fascinated by the tale itself, its many possibilities and reincarnations in art over time. These I will bring before you in the coming days.
So far, I’ve been quoting Robert Browning’s excellent rendition, The Pied Piper of Hamelin (1905).

Once more he stept into the street;
And to his lips again
Laid his long pipe of smooth straight cane;
And ere he blew three notes (such sweet
Soft notes as yet musician's cunning
Never gave the enraptured air)
There was a rustling, that seemed like a bustling
Of merry crowds justling at pitching and hustling,
Small feet were pattering, wooden shoes clattering,
Little hands clapping, and little tongues chattering,
And, like fowls in a farm-yard when barley is scattering,
Out came the children running.
All the little boys and girls,
With rosy cheeks and flaxen curls,
And sparkling eyes and teeth like pearls,
Tripping and skipping, ran merrily after
The wonderful music with shouting and laughter.

Posted above is Ingrid Bergman’s take on Browning’s verse. Bergman's reading is charming and endearing, and all the more deliciously creepy for that.