"marathon day" is a correct phrase and can be used in written English.
It is typically used to refer to the day of a physical marathon race. For example, "Next Saturday is the marathon day, and I'm so excited!".
For 30 years, Bishop Loughlin High School's marching band has set up at the very same spot on marathon day.
As in London, the production will arrive at Broadway's Lyric Theater in two parts, which audiences can watch over two days or in a single marathon day.
It can be a marathon day or two when I put the Puzzle Corner together.
An ideal marathon day would be 50°F; every 10 degrees slows things down.
But in New York City on marathon day, finishing at all qualified as triumph.
The agenda was always the same, and it was a marathon day.
After a marathon day six in Melbourne, I think I might need one too.
Experts suggested a variety of ways to prepare for unexpected heat on marathon day.
This year's marathon day dawned gusty, cloudy and wet, but that did not deter the crowds.
Jim Furyk quietly pumped his fist when his final putt on a marathon day disappeared in the cup.
Judge Alito had a marathon day of 14 such meetings on Thursday, and they will resume next Tuesday.
