"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".