Not necessarily. People like to shop online at any convenient time be it weekdays or weekend.
One of the reason why people buy more on holidays is because most of the shopping sites offer big discount on holidays during special occasions and festivals.
Sunday is good to buy if there is a family decision involved, otherwise individuals will make a decision with their peers & friends before buying it. Sunday is good for Home item purchases.
I love to shop anytime and any day. Moreover, I'm comfortable with both online as well as shopping from a physical store. The only thing that is of the biggest concern is that I should have enough money to spend on my shopping.
I prefer the day when there is a discount or some good offer going on. It does not matter what day is it. I am in a look to grab the best offer on something that I need to buy.