It's weird, I know. My youngest (5) does the same thing and it's usually right around the full moon. I have a lunar calendar and it's really handy as I can tell from the calendar and I can see his temperment changing and building the week before. As far as I know, there's no scientific reason for it, but I also know that during the full moon is when a lot of mental hospitals have more staff as outbursts and behavior become more extreme.
What I do is try to give him more positive attention the week before so it "tempers" his temper. It reduces the severity, but he still has his breakdowns. What I do is tell him that he is going to be in his room for 5 minutes of quiet time. If he screams, cries, yells, throws anything, misbehaves in any way, the time starts over. I have a timer so he knows how long he has. That has reduced his meltdowns from 2 hours to about 20 minutes.
All I can say is good luck, and no matter what, don't give in!!! :) If you do, it'll be way worse the next time.