The Supplemental Nutrition Assistance Program (SNAP) is the largest nutrition assistance program in the United States and is a vital part of the social safety net. The monthly lump-sum benefit distribution schedule has led to higher food expenditures and consumption shortly after the date of benefit receipt, and a diminishing food expenditures and associated food consumption pattern towards the end of the benefit month. This has been found to have adverse effects on beneficiaries' dietary outcomes. In this paper, we investigate food purchasing patterns of SNAP households over the month using data from the National Food Acquisition and Purchase Survey (FoodAPS), a newly developed nationally representative survey, and its geographic component. We ask the research question: do SNAP recipients pay more for otherwise similar food at the beginning of the benefit cycle, and substitute less costly food as the month proceeds? We found that the unit cost of food bought declines significantly over the benefit cycle when households make purchases using SNAP benefits. Our results shed light on the optimal policy design of food assistance programs through a better understanding of program participants' food-purchasing behavior.