Season 12 End & Season 13 Start

Season 12 will end on the following dates:

North America: Sunday, February 11 @ 5:00 p.m. PST
Europe: Sunday, February 11 @ 5:00 p.m. CET
Asia: Sunday, February 11 @ 5:00 p.m. KST

Season 13 will begin about two weeks afterward on the dates below:

North America: Friday, February 23 @ 5:00 p.m. PST
Europe: Friday, February 23 @ 5:00 p.m. CET
Asia: Friday, February 23 @ 5:00 p.m. KST

Source: https://us.battle.net/d3/en/blog/21492188

Check out the Season 13 overview here.