Since early times, Christians in the West adopted the practice of marking their foreheads with ashes, to commence the period of Lent, a time of reflection, prayer and fasting that lasts six weeks, before the celebration of Lord Jesus’s death and resurrection, called Easter.
These ashes represent among other things, the fact that humans are called to eventually die and return to dust (Genesis 3:19; 18:27); and also the ashes people used to put on themselves as a sign of repentance before God (Matthew 11:21; Joshua 7:6; Job 42:6; Jonah 3:6). Those who take on this pious practice, also practice a perfect fasting on that day, which is abstinence of food and sex for the whole day until the evening.
These days, the practice is kept by most western traditional Churches as a spiritual preparation for Easter. However, whether it is maintained or not, all Christians are called to reflect on their lives and unite themselves spiritually with our Lord in the 40 days of fasting He endured in the desert (Matthew 4:1-2), and reflect in the meaning of his crucifixion and resurrection as a propitiatory offering to the Father for the salvation of humanity.