...so if you see someone with a smudge on their forehead don't tell them they have something on it!
The following excerpt is from Wikipedia:
Ash Wednesday, according to Christian tradition, in the Western Christian calendar, is the first day of Lent and occurs 46 days (40 days not counting Sundays) before Easter. It is a moveable fast, falling on a different date each year because it is dependent on the date of Easter. It can occur as early as February 4 or as late as March 10.
Ash Wednesday derives its name from the practice of placing ashes on the foreheads of adherents as a sign of mourning and repentance to God. The ashes used are typically gathered after the palms or Palm Crosses from the previous year's Palm Sunday are burned. After the ceremonial burning of the remains of the palms, the ash is mixed with a small amount of water to create a more adhesive substance.
This practice is common in much of Christendom, being celebrated by Catholics, Lutherans, Methodists, Presbyterians, and Anglicans.
At Masses and services of worship on this day, ashes are imposed on the foreheads of the faithful (or on the tonsure spots, in the case of some clergy). The priest, minister, or in some cases officiating layperson, marks the forehead of each participant with black ashes in the shape of a cross, which the worshipper traditionally retains until it wears off. The act echoes the ancient Near Eastern tradition of throwing ashes over one's head to signify repentance before God (as related in the Bible).