"Each year, around this time, the Zaford clan takes a break from their war with the Hodunk bandit family to celebrate their good fortune and honor those that have passed on, such as that time Scooter buried their son alive in a shallow grave." - Gearbox Website
New skins for all 5 characters as well as a "Chulainn SMG" that will be the same level of the character that you first load, after you enter the Shift Code.
Here are the codes..
PC / Mac Zaford Gear SHiFT Code: KTCBJ-C9RTC-BSFTJ-33TT3-FBWHF
Xbox 360 Zaford Gear SHiFT Code: KJCJ3-RCWC6-5K9BF-WFBT3-3WFKK
PlayStation 3 Zaford Gear SHiFT Code: CTK3B-99K3J-XXKW3-ZTWTJ-HJH5J
They won't be available after the weekend, check out Gearbox Software's Official site for exact timings and more info below.
Source: Gearbox Software