I absolutely avoid going to Costco on Sunday afternoon.
I may go to Costco Sunday early morning, but I make sure that I get out around noon. That's when a wave of church people coming to Costco. I believe Sunday after noon is the busiest time at Costco.
On Sunday afternoon, I see a lot of people wearing nice church clothes. Some people look rich, and some look poor. But they are all good looking. In many cases, they come to Costco with number of family members.
I usually go to Costco after work on the weekdays. I enjoy slow traffic inside Costco that way. Also, I spend little or no time waiting at gas station line.