USA Today reports that Best Buy had the biggest day ever for its four-year old trade-in program on June 1. That's when the electronics retailer gave customers the opportunity to trade in iPhone 4 and 4S models and get a "free" iPhone 5 in return. The company is about to re-launch the promotion on Friday and keep it going for 9 days.

Under the program, customers who trade in an older working model iPhone 4 or 4S can earn a credit of up to $150, which can be used to cover the purchase of a new 16 gigabyte iPhone 5, at the retailer's current $149.99 price. The price factors in a $50 instant rebate that stays in effect until June 29.

To qualify, buyers must be eligible for an iPhone upgrade with their wireless carrier, and are still on the hook for a new two-year data plan, plus sales tax of around $35. You can get a phone from Verizon Wireless, AT&T or Sprint.

See if you qualify at http://www.bestbuy.com/tradein