This is the first day of Lent in the Christian religion. It is approximately 40 days before Easter when Jesus Christ rose from the dead. The number 40 days comes from passages in the Bible where Jesus went to the desert and prayed and fasted for 40 days. The thinking is that Christians can stand to fit into their schedule some small amount of prayer and fasting, too.
The custom of ashes comes from Biblical Times where penitents used ashes to signify their remorse of their sins. It also ties into the idea of “ashes to ashes, from dust to dust”. In the end, we’re all going to some spiritual destination in the afterlife or if religion is just superstition, we’re going to just stop existing.
The custom of putting on a sprinkling of ashes on one’s forehead grew in the first millennium of Christianity and was ratified by Pope Urban II in 1091. Protestants had and have varying beliefs and practices on this.
I like the idea of this holiday because it reminds us of the big picture in life.
Further information: Wikipedia.