By Missouri state law, the sales tax holiday begins on the first Friday in August and continues through the following Sunday. In 2014, the three-day holiday begins at 12:01 a.m. on Friday, Aug. 1, and runs through Sunday, Aug. 3. Certain back-to-school purchases, such as clothing, school supplies, computers, and other items as defined by the statute, are exempt from sales tax for this time period only.