What is Ash Wednesday? (Nathalie asked why there were people getting ashes on their forehead at a METRO station)

Ash Wednesday is an important day for Roman Catholic Christians. This video explains it.
Ash Wednesday is a time to remember and regret any bad things you did.  It's a time to remember that you are going to die and that you want to be a better person right now.

There is an organization called "ashes to go" run by the Episcopal Church (Anglican (British English) Church in the United States)  Ministers impose ashes at the METRO, in the streets, and in coffee shops.  This custom has only been around for about five to ten  years.  Many conservative Christians dislike it.