The exact timing for the seasons will vary a bit based on where you live. Here's the timetable posted on the Diablo 3 forums:
"All Seasonal Leaderboards will be wiped and you will be unable to attain new Leaderboard ranks until the next Season begins," Blizzard said previously about the season transition. "The results of the previous Seasons Leaderboards will remain available for viewing in-game. Once the new Season begins, a handy drop-down menu will allow you swap between the current and previous Season records. Dont forget that you can also view current and past Leaderboards right here on our website."
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