When did Judas know that he was going to betray Jesus?

He betrayed him on the Thursday, the night of the last supper. Matthew 26:2, together with v14-15 records that, it was two days earlier, on the Tuesday, when he met with the priests who wanted to kill Jesus.

2Ye know that after two days is the feast of the passover, and the Son of man is betrayed to be crucified. 14Then one of the twelve, called Judas Iscariot, went unto the chief priests, 15And said unto them, What will ye give me, and I will deliver him unto you? And they covenanted with him for thirty pieces of silver.