Harry Potter and the Goblet of Fire Easter Egg - The Ball Band

The band that Dumbledore books for the Yule Ball during the Triwizard Tournament is called the Weird Sisters (first found on page 390 of the hardcover addition), a nod to William Shakespeare's play "Macbeth". In the play Macbeth the three witches who give Macbeth prophecies are known self described as the "Weïrd Sisters". (Act 1. Scene 3. Line 33)
The band also appears in other books in the Harry Potter series.