Diablo III's 2.3 patch changed the game a fair bit, especially with the addition of Kanai's Cube. Blizzard want you to be able to take advantage of the cube's transmutation abilities, so they're giving out double Caches for any full set of bounties you complete, for this weekend only.
This applies to both Horadric and Bonus Caches, so make sure you get those bonus acts for extra rewards.
This bonus period begins in the Americas region this Friday, September 25, at 12:01 a.m. PDT and ends on Sunday, September 27 at 11:59 p.m. PDT.
Obviously, to take part in the double Cache festivities, you need to have downloaded the 2.3 patch and be all up-to-date. So if you haven't already done so, now would be a good time. Everyone loves a bit more Cache...s.
Times will be the same for whatever region you live in, so in the UK it will be Friday, September 25, at 12:01 a.m. GMT and ends on Sunday, September 27 at 11:59 p.m. GMT, etc. And you thought you were going to sleep tonight.