Just a day after details of T-Mobile's upcoming Magenta Deal Days sale leaked online, the folks at Magenta have made the promotion officially official. Dubbed the Magenta Deal Days Zero Down sale, T-Mobile says that the promo will allow customers to pick up any device for a net cost of $0 after signing up for a qualifying Unlimited Value and Unlimited Nationwide 4G Data (Unlimited, 5GB or 10GB) plans and sending a mail-in rebate. The sale will run from September 21 through September 30 and will be open to new and existing customers, though T-Mobile notes that standard migration fees will apply to Classic customers who switch to a Value plan and are less than 18 months into their contract.

It's also worth mentioning that since customers taking advantage of this sale will be dealing with a mail-in rebate, they'll have to make a down payment at the T-Mobile store when initially buying their device. Also, a monthly Equipment Installment Plan will be applied for 20 months since the deal involves Value plans. T-Mobile has provided examples for how much customers can expect to pay in-store for some devices. The Samsung Galaxy S III features an out-of-pocket down payment of $149.99 but also includes a $150 down payment, which T-Mo explains will bring the net down payment to $0, and then there's a $20 per month EIP. The new Samsung Galaxy S Relay 4G has an out-of-pocket down payment of $99.99, a mail-in rebate of $100 and a $15 EIP.

Since this deal will run from September 21 through September 30, interested T-Mobile customers will have plenty of time to take advantage of it. It's good to see the Galaxy S III included in the promotion, though I'm sure that there are some folks out there that'd prefer to avoid having to deal with the mail-in rebate and simply get the $0 down payment price in-store. If you're a T-Mobile subscriber that's planning to stick with Magenta and is eligible for a new device, though, the Magenta Deal Days Zero Down sale may still be worth looking into. We've got T-Mobile's full announcement below.

T-Mobile Helps Customers Fall Into Savings

From Sept. 21–30, get any device, including the Samsung Galaxy S® III, for $0.00 down

Bellevue, Wash. — Sep. 18, 2012

2012-09-18 16:35:04

Prices are falling at T-Mobile® this September during the “Magenta Deal Days” Zero Down promotion, making staying in touch more affordable than ever. Starting Sept. 21, customers can purchase any device, including the Samsung Galaxy S® III, for $0.00 down payment on qualifying Unlimited ValueTM and Unlimited Nationwide 4G Data plans after mail-in rebate card.

“Our customers continue to appreciate our ‘Magenta Deal Days’ offerings because they showcase our commitment to giving all our customers the best 4G experiences,” said Andrew Sherrard, senior vice president of marketing, T-Mobile USA. “With back to school in full swing, we’re glad to offer another great deal to keep families in touch.”

The Samsung Galaxy S III is one of T-Mobile’s fastest 4G (HSPA+ 42 technology) smartphones. With an intuitive design, the Galaxy S III enables customers to effortlessly share photos, videos and more to a compatible TV or other Galaxy S III devices. Featuring a beautiful 4.8-inch HD Super AMOLED™ screen, the Galaxy S III packs a powerful entertainment experience, which allows customers to watch HD movies and TV shows, download apps, and securely browse the Web.

New customers will be eligible for this deal when they sign a two-year agreement on any qualifying Unlimited Value and Unlimited Nationwide 4G Data plan (Unlimited, 5GB or 10GB). Existing customers also can take advantage of this offer.1

Data-hungry smartphone customers can choose the ultimate worry-free experience with T-Mobile’s new Unlimited Nationwide 4G Data plan, featuring no data caps, no speed limits and no overages, as well as fast, dependable nationwide 4G coverage. For more details about T-Mobile’s Unlimited Nationwide 4G Data plan, visit http://www.T-Mobile.com/unlimited-data-plan.

The “Magenta Deal Days” promotion will be available at participating T-Mobile retail stores nationwide. For more information about T-Mobile’s “Magenta Deal Days” promotion and details about qualifying plans and services, visit http://bit.ly/NldCS8.

1 Standard migration fees will apply for existing Classic customers who switch to a Value plan and have less than 18 months of contract tenure.

2 Equipment installment plans: On approved credit and 0 percent APR. Down payment and unfinanced portion required at purchase. Remaining balance paid in 20 monthly installments.