“It's just been one of those days.”
Today you went to work looking messy and scruffy because you woke up late, spilled some coffee on yourself, and had a few other accidents. A coworker is asking if you're OK because of how you look. You offer this explanation for your appearance.
It's just been one of those days.
When used in this way, "just" means "very" or "completely". It can be used with positive or negative adjectives:
This was just a horrible, horrible idea!
"It's been one of those days" is a set expression that people say when:
- they've had a bad day
- they've been really busy all day
- a lot of unlucky accidents happened
Any of these can be described as "one of those days".
People often use this phrase at work. For example, if someone seems to be stressed out at work you can say to them:
One of those days, huh?
If your coworker tells you that they've "had one of those days", you don't really have to ask them about it. "It's been one of those days" expresses the feeling that bad days sometimes happen and can't be avoided.
In this phrase, "It's" is an abbreviation of "It has".