What does letter of the day mean in ACT language?

"Letter of the day" is not a term used in the ACT. The ACT is a standardized test for college admissions, and it focuses on subjects like English, mathematics, reading, and science. There is no concept of a "letter of the day" within the exam's structure or scoring.

It's possible you're thinking of a different context, like:

* A classroom activity: Some teachers might use "letter of the day" as a fun way to introduce a specific letter of the alphabet and its sound each day.

* A game or puzzle: There might be a game or puzzle that features a "letter of the day" as a central element.

If you can provide more context, I can help you understand what "letter of the day" might refer to.