Four days to the season 3 end on Sunday, then five days to finish inventory+character cleanup before season 4 starts Friday. Patch would be on the Tuesday in between.
Season 3 will be ending at the following dates and times for each region:
North America: Sunday, August 23 @ 5:00 p.m. PDT
Europe: Sunday, August 23 @ 5:00 p.m. CEST
Asia: Sunday, August 23 @ 5:00 p.m. KST
(Patch Tuesday Aug 25)
Season 4 will be starting shortly after at the following dates and times for each region:
North America: Friday, August 28 @ 5:00 p.m. PDT
Europe: Friday, August 28 @ 5:00 p.m. CEST
Asia: Friday, August 28 @ 5:00 p.m. KST
http://us.battle.net/d3/en/blog/19842943/season-3-ending-soon-8-7-2015#interseason