Not a damn thing.
If something is virtuous, it should be practiced as often as it is required. if something is vile, it should be avoided unceasingly. To mark a day on the calendar to give up the vices is the same as giving yourself permission to practice that which you believe is morally, spiritually or physically harmful to you the rest of the year.
This sort of stupidity leads to exactly what the Catholic church has become: Decadence, wrapped in ceremony, tied with a ribbon of piety. Ash wednesday becomes a reason for Fat Tuesday, which becomes a reason for Mardi Gras. The anticipation of a minor performance of piety becomes the excuse for a major explosion of decadence.
Practice virtue every day. Eschew vice every day. Even if you are Catholic.