"marked day" is correct and usable in written English.
You can use it to refer to an important event or date that has special significance. For example, "Today marks the day we celebrate our country's independence."
Sunday Dec. 4 marked day 50 of Occupy Phoenix.
December 11 marked day four of the Student City revolt.
Activists keep count— Monday marked day 648 since West's death.
They will release one new entry a day -- the most recent entry is marked "Day 16".
Monday marked day 38 of the search for the Boeing 777, which disappeared March 8 en route from Kuala Lumpur to Beijing with 239 people aboard.
Columbus Day marked day 24 of the Occupy Wall Street protest and the first that activists could bring their kids to Zuccotti Park without taking them out of school.
"There may have been some nice, pleasant times... but for the most part, death, sickness, disease, accidents, bad weather, loneliness, strenuous hard work, insect-infested foods, prowling predatory animals, and countless inconveniences marked day-to-day existence".
Human social contact patterns show marked day-of-week variations, with a higher frequency of contacts occurring during weekdays when children are in school, and adults are in contact with co-workers, than typically occur on weekends.
A key development marked Wednesday's announcement.
John Lennon's 75th birthday to be marked Friday in Hollywood.
Professionalism, elegance, and discipline marked Friday's performance.
