Because on Wednesday Judas decided to betray Our Lord.
Gospel from Mark 14, 1-11 (King James' Bible):
After two days was the feast of the passover, and of unleavened bread: and the chief priests and the scribes sought how they might take him by craft, and put him to death.
But they said, Not on the feast day, lest there be an uproar of the people.
And being in Bethany in the house of Simon the leper, as he sat at meat, there came a woman having an alabaster box of ointment of spikenard very precious; and she brake the box, and poured it on his head.
And there were some that had indignation within themselves, and said, Why was this waste of the ointment made?
For it might have been sold for more than three hundred pence, and have been given to the poor. And they murmured against her.
And Jesus said, Let her alone; why trouble ye her? she hath wrought a good work on me.
For ye have the poor with you always, and whensoever ye will ye may do them good: but me ye have not always.
She hath done what she could: she is come aforehand to anoint my body to the burying.
Verily I say unto you, Wheresoever this gospel shall be preached throughout the whole world, this also that she hath done shall be spoken of for a memorial of her.
And Judas Iscariot, one of the twelve, went unto the chief priests, to betray him unto them.
And when they heard it, they were glad, and promised to give him money. And he sought how he might conveniently betray him.
The first day of Passover was Friday so it was Wednesday when this took place.