Work of Art

'When Shall We Three Meet Again?'
I passed by this large cutout sign, sitting on a wooden box marked The Witches, in an Edinburgh park ~ I don't know what it means or refers too, does anyone know?
Yes thank you blippers for putting me right ; here's a quote from Macbeth


Macbeth is one of William Shakespeare's great tragedies. There's murder, battles, supernatural portents, and all the other elements of a well-worked drama. Here is the quote from Macbeth.
"First Witch: When shall we three meet again
In thunder, lightning, or in rain?
Second Witch: When the hurlyburly's done,
When the battle's lost and won."
- William Shakespeare, Macbeth, 1.1