For Christians, it is not a certain day. Our sins are forgiven when we ask God for forgiveness. It's an ongoing thing since none of us are perfect. The Bible says that when we truly repent, our sins are forgotten forever.
You threadjackers (and you know who you are) owe me for a new keyboard! I read your posts and promptly sprayed half chewed Gobi Aloo all over the screen. I'm going to be smelling curry everytime I write an email now... :-X