Our friends at Wikipedia explain it:
In Western Christianity, the Wednesday before Easter is sometimes known as “Spy Wednesday”, indicating that it is the day that Judas Iscariot first conspired with the Sanhedrin to betray Jesus for thirty silver coins.
This event is described in the three Synoptic Gospels: Matthew 26:14-16, Mark 14:10-12, Luke 22:3-6.
The Sanhedrin was gathered together and it decided to kill Jesus, even before Pesach if possible. In the meantime, Jesus was in Bethany, in the house of Simon the leper. Here he was anointed on the head by Mary, the sister of Martha and Lazarus, with very expensive ointment of spikenard. Some of the disciples were indignant about this; the oil could have been sold to support the poor. Judas went to the Sanhedrin and offered them his support in exchange for money. From this moment on Judas was looking for an opportunity to betray Jesus.
* Poland: children traditionally hurled an effigy of Judas from the church steeple. It was then dragged through the village, pounded with sticks and stones and what was left of it was drowned in a nearby pond or river.
* Czech Republic: the day is traditionally called Ugly Wednesday, Soot-Sweeping Wednesday or Black Wednesday, because chimneys used to be swept on this day, to be clean for Easter.
* Malta: this day is known as L-Erbg?a tat-Tnieber (Drums’ Wednesday), in the past children went to the parish church and drummed on the chairs to make the sound of thunderstorms.