2016 registration was (I think) on June 1st. I remember what I was doing that day and it was the Wednesday of that week which would be June 1st. I was at a small business with crummy wifi and tried for the better part of an hour to login and join and couldn't get in, and didn't get picked in the lottery after that was held to makeup for the site crashing all day on the 1st.
Edit: yes it opens every year, at least that has been the case so far. I have never made it in, only trying for real this past registration period. There are a limited number of spots and I don't think former members get any special treatment ( somebody can correct me if I am wrong) so everybody has to try and get in at the same time. For 2016, the registration site crashed all throughout the day and only like 20% or something (that I heard) of the slots got filled before they closed it down because it was such a mess. It was reported that they would offer the remaining spots via a lottery of all registered accounts on the saints and sinners web site, but I didn't get an offer and haven't heard much about it after that.