The only fast day commanded in the bible, Yom Kippor, or the Day Of Atonement. Once a year you can get out from the consequences of your sins if your heart is sincere and you are serious about never doing it again. Of course, only God knows your future and if you truly turn away from your sins you will not have to suffer the punishment for that sin. You can also gain forgiveness for your pagan family members for a fresh start.