After Season 6 ends, there will be one week of downtime where no rated matches can happen.
Season 7 will begin after this week of downtime, at which time all team and personal ratings will be wiped, along with your arena points. According to the announcement matchmaking ratings will remain (as well as honor points). None of this should come as too much of a surprise, as we've known about it all for a while.
This roughly places the start date for Season 7 at Tuesday September 1st. However these things can change, so just pay attention to the news and watch out for the exact date.