This app was mentioned in 1 comment, with an average of 2.00 upvotes
https://play.google.com/store/apps/details?id=net.iboxmanstudio.mart
One thing you will learn is the constant question of -- Is Emart/Costco/HomePlus/Lotte open today? People always forget which Sunday it is (Sometimes Wednesday)? The app is just a simple one that allows you to see the individual stores and if they are open or closed. It is just in Korean.
In general the big stores are closed on the Second and Fourth Sundays of the month. Plus sometimes on major holidays like Chusok and New Years Day.