The phrase "busiest day of the week" is grammatically correct and usable in written English. You can use it to refer to the day of the week with the most activity or activity level. For example, "Monday is usually the busiest day of the week for our retail stores.".
Sunday is the "busiest" day of the week.
"The busiest day of the week was Thursday," she said, "when people were going to the cow sale".
All films faced the challenge of Bonfire night falling on a Saturday – usually the busiest day of the week for cinemas.
Fridays, often the busiest day of the week, were taken up with constituency visits, surgeries and party meetings that would sometimes continue until late in the evening.
British Airways, the largest carrier at Heathrow, says Thursday will be its busiest day of the three weeks around Easter, when it will fly 2.4 million passengers.
"That was the busiest day of our week".
About 30 trains traverse this track on the busiest day of week.
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Cristina Valenza
Retail Lead Linguist @ Apple Inc.