The phrase "a single day when the" is correct and usable in written English.
It can be used to refer to a specific day that is significant or noteworthy in some context.
Example: "There was a single day when the entire city came together to celebrate the festival."
Alternatives: "one particular day that" or "a specific day on which".
I have been in this position for over 3 years and I can say, without fear of contradiction, that no two days have ever been the same and there has not been a single day when the job has not been interesting and challenging.
But there hasn't been a single day when the trash wasn't cleared outside my kitchen door.
In South Florida, eight seniors died in a single day when the air conditioning went out at a nursing home after Hurricane Irma knocked out the power.
The Globo TV network, citing security officials, reported that 200 cars were stolen in Vitória on a single day, when the state average is 20.
During the period from March to October , 1996 there was not a single day, when the daily averaged intensity of 1.6 3 MeV protons exceeded the level of 1 proton/(cm2 sr s MeV).
Its bond holdings of government-sponsored enterprises, estimated by credit rating agencies at $340 billion, rose in value by billions of dollars in a single day when the Bush administration made explicit the government guarantee of Fannie Mae and Freddie Mac bonds, causing their interest rate spreads compared with Treasury bonds to narrow by 5 to 35 basis points within hours.
A sensor on the always-congested Brixton Road, one of many monitored by King's College London, recorded a staggering 17 hours in a single day when NO2 concentrations exceeded the "safe" 200 micrograms per cubic meter limit set by the World Health Organization.
It's possible that there's simply less discrepancy between Christmas Day and the rest of the year because people are more used to the concept of app stores, and more likely to buy mobile software throughout the year than on a single day when surrounded by more tech savvy relatives who can guide them through the process.
Isaac treats himself to vanilla custard and orange sherbet while Greg Kohr, the shop's owner, explains how his grandparents invented frozen custard in the 1930s selling 18,000 cones in a single day when they unveiled it in Coney Island.
The study was conducted in one location on a single day when trainees may be expected to be particularly anxious about starting work.
"During the day, I was really lucky: I never spent a single day when my wife or one of my kids wasn't there.
