This one is a little bit of tricky because I do not have a context.
There's even a saying that says "Every day is arm day".
Does it mean workout day?
Without context, that's the first thing I think of. [Body part] day is the day you target and work out your [body part]. On arm day, you work out your arms. They often say, "Don't skip leg day."
However, it could easily be something else that has to do with your arm, or [body part], with a particular context in mind.
"Every day is [X] day" is a well-known formula into which you can plug almost any activity that you consider so important you must do it every day.
Examples:
- Every day is pizza day
- Every day is a workout day
One particular case that's a common phrase:
- Every day is a school day
This is a metaphor meaning the same as, "You learn something new every day."
Another possibility is that it could be short for “Armistice Day,” which is an older name for Veterans Day. Although this is not a standard abbreviation for this term, Veterans Day just occurred (November 11), so without any context, this is what I thought of when I read the quote.