Why do Christians apply a small dusty cross on their foreheads once a year? They're celebrating Ash Wednesday, which for many also marks the start of Lent, a 40-day period of penance, fasting and reflection leading up to Easter. 

Lent requires believers set aside time every year to recognise the period and its connection to Jesus Christ and his time in the wilderness.

If you would like to learn more you can go to these helpful websites: 

'Remember that you are dust': The history of Ash Wednesday | Sky HISTORY TV Channel
Ash Wednesday - Wikipedia