Build 2013 was held from June 26 to June 28, 2013 at the Moscone Center (North and South) in San Francisco. The conference was primarily used to unveil the Windows 8.1 update for Windows 8. Each attendee received a Surface Pro, Acer Iconia W3 (the first 8-inch Windows 8 tablet) with a Bluetooth keyboard, one year of Adobe Creative Cloud and 100GB of free SkyDrive storage.
Here is the new start screen of Windows 8.1
BUILD 2013 Day 1 Keynote by Steve Ballmer and Julie Larson-Green. They showed off Windows 8.1, a ton of great Windows and Windows Phone devices, new Bing and Xbox One features and more.
BUILD 2013 Day 2 Keynote by Satya Nadella, President of Server and Tools Business at that time. This keynote is filled with updates about Windows Azure, Visual Studio, ASP.NET, and Office 365.
BUILD 2013 had more than 200 sessions. You can watch them here.