On this day, ash is placed on the forehead of believers to remind us that we are made from dust and dust we shall return to and also to remind us that we are mortal, sinful and in need of repentance.
When jonah brought the message of Gods proposed wrath to the people of nineveh, the people of nineveh sorth Gods forgiveness by praying, fasting, putting on sack clothes and pouring ash on their body. Gods response to their remorse was total forgiveness and withdrawal of the punishment prepared for them. Hence us christians have to make sure that this ash, even when is physically gone, should spiritually remain on our foreheads to constantly remind us of our nothingness before God, boost our humility and get us closer to God. Happy Ash wednesday.