The official availability date for the Samsung Galaxy S6 is April 10th, but T-Mobile cares not for the workings of the calendar. Pre-orders made on Friday with overnight shipping are currently on-track for Monday deliveries, and a few Edge devices have already shown up for those who selected Saturday delivery.
The availability of Saturday delivery depends on when an order is placed and where your shipping address is. Most people only have the option of doing standard overnight, which is $25 for upgrades. Still, there are two people on XDA who have posted pictures of their shiny new Galaxy S6 Edge devices. No standard Galaxy S6 units have been confirmed as delivered yet, but it's possible those have also shown up.
If you're still on the fence about the Galaxy S6, you should be able to order from T-Mobile for Tuesday or Wednesday delivery at this point. It'll cost you a little extra, but you'll have the phone almost two weeks before most everyone else.