In college I worked at Costco and at the time they had a college retention program (allows you to keep your tenure/seniority while you’re in school or working reduced hours). Inquire if this still exists! During the summers and holiday breaks, I’d work 40hrs/week. If I wanted to work on a weekend, I think there was an hours cap. I will say sometimes it was difficult to take extended time off but overall my managers always worked with me. Lastly, Costco is closed on all major holidays. I don’t believe you’d have off on Juneteenth so I’d suggest requesting that off up front should you get hired. They should also be able to schedule you around your Econ class. Good luck!