Diablo 3's second season will be starting later this month. Today Blizzard announced the full schedule for Season 1's end and Season 2's launch.
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:
Season 1 End
"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 Season’s 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. Don’t forget that you can also view current and past Leaderboards right here on our website."
You'll then have to wait a week-and-a-half for Season 2 to kick off:
Season 2 Start
However, Season 2 will bring new rewards. If you reach max level in Season 2, you'll receive boots and pants for transmogrification. These items match the helm and shoulders you could earn in the first season. Furthermore, you'll get other additional bonuses by hitting certain Achievement points totals throughout the season:
Conquests, the Season-specific Achievements, are also getting some changes. Blizzard added new Conquests like Worlds Apart that challenge players to kill several bosses in 20 minutes. The Conquests that merely asked players to level to 70 in normal or hardcore mode, meanwhile, won't be returning. The power levelers who found ways to hit level 70 within hours just to top the leaderboards probably drove that change.
Seasons are only available on the PC and Mac versions of the game. Blizzard won't be releasing this feature on PS3, Xbox 360, PS4 or Xbox One.
"A large part of the fun of Seasons is the entire community rerolling heroes together at the same time," Blizzard said last fall. "The offline and disconnected nature of console does not support this style of play. As a result, we do not currently have plans to implement Seasons on console."