This weekend only, for each full set of bounties Diablo 3 players complete, they will be rewarded with double caches.
This applies to both Horadric and Bonus Caches as well.
Per Blizzard, it starts: “Friday, September 25, at 12:01 a.m. PDT and ends on Sunday, September 27 at 11:59 p.m. PDT.” Or whatever your timezone happens to be.
So, in other words, since Blizzard’s wording is a bit “off” – come midnight (you know, since today is Thursday and stuff) on the west coast you can start earning double.
It will then end Sunday night on the west coast – Monday morning if you are on the east coast.
Hope that is clear for you.
You can find out more information on the Diablo 3 Double Cache weekend over on Battle.net.