Costco closes at 6pm on Sundays. They close at this time on Saturdays as well but stay open until 8;30pm on Monday through Friday. Costco is a wholesale warehouse discount store that requires a membership to shop.
Costco is able to bring its members some of the best possible prices on bulk items. Another example of a warehouse wholesale store that is much the same as Costco is Sam's Club. Sam's Club is an affiliate of Walmart.