This is the 5664th most frequent Chinese word.
十八日
"十八日" means "the eighteenth day."
Here, '十八日' refers to a specific day in the month, describing the relationship of the date to the week.
十八日是星期四。
The eighteenth is Thursday.
In this case, '十八日' indicates a specific date in a historical context, adding significance to the day.
去年八月十八日是个难忘的日子。
Last year's August 18th was an unforgettable day.
In this sentence, '十八日' specifies a date within a month, acting as a time expression.