T-Mobile's Samsung Galaxy S4 starts receiving Android 4.3 today
Besides sending Android 4.3, the update will make the phone compatible with the Samsung Galaxy Gear smartwatch. It also adds Samsung KNOX to the device. This offers a number of applications that are fenced in to prevent a security breach. Tapping the icon offers a secure browser, contacts list, email and calendar applications.
Considering the large size of the file at 781MB, your best bet is to make sure you have a Wi-Fi signal when downloading the update, and your battery should be at least half filled. These types of updates are usually sent out on a staggered basis, so if you don't get the update today, or tomorrow, it will come soon.
source: T-Mobile via AndroidCentral