I'm wondering if it's possible to extend the sale's end to the weekend following the 15th, which for a huge percentage of customers means an additional paycheque at their disposal -- especially as most people have to pay the majority of their bills at the beginning of the month, so the mid-month paycheque is usually the only one with any discretionary spending associated with it :-).
In the US, most employers issue bi-weekly paycheques, and due to recent banking law changes (maybe a year or so back), they generally don't show up as available funds until Friday during the day (they used to show up Wednesday at midnight, or Thursday during the day). The end date of 15 January effectively cuts off a lot of potential sales following the Christmas holiday.
Just a suggestion.