Varies with your server. Even servers one week, odd servers the next. You must have completed the first (level 20) job change to sign up. Go to either the Dusk Pristess, or the Priest of Dawn to pick your side. If you have completed the second job change, you will need 50k or a certificate to pick Dawn unless your clan owns a castle.
Both the Priestess and the Priest will tell you if you can sign up this week on your server.