Ash Wednesday is the first day of Lent--the 46 days before Easter. Lent reminds us of the 40 days Jesus spent in the wilderness fasting and praying after his baptism. The extra 6 days in Lent are for the Sundays during the time period.
Ash Wednesday gets it name from the ashes that are placed in the shape of the cross on people's foreheads. Often as part of the process, a minister may say, "Repent, and believe the Gospel" (Mark 1:15) or "Remember that you are dust and to dust you shall return" (Genesis 3:19).
In Biblical times, ashes were used as an expression of grief or repentance. Today, we wear the ashes to help us remember our own need to acknowledge our sins and change our ways. Lent is also a time to remember the grief that came with Christ's death on the cross (leading to his resurrection on Easter).