I don't know exactly when we'll go but we really must visit them one of these / one of those days.
When should we use "one of these days" and "one of those days"?
One of these days
One of those days
These are idioms.
The former means sometime in the near future. So you can say "we really must visit them one of these days".
The latter (one of those days) means a bad day; a day when everything goes wrong.
I missed breakfast, got late to work, and got caught in the rain at lunchtime - it was one of those days! (The Free Dictionary).
It looks like it's going to be one of those days (McMillan).
So you shouldn't use this idiom in your sentence presented.
The short answer.
The two phrases are idioms.
So if you want to visit them in the near future, but you're not sure when, then use
I don't know exactly when we'll go but we really must visit them one of these days.
The long answer.
The two expression have them meanings as above, but they can also take on various meanings depending on the context.
For example "one of these days" can also mean
[1.] someday; in some situation like this one
One of these days, someone is going to steal your purse if you don't take better care of it. You're going to get in trouble one of these days.
It can also be used to refer to the past. For example, if you are looking at a calendar from last year, you started point at the month of May. As you tried to recall a particular event, you say "I know it happened one of these days, but I can't remember which one."
As for "One of those days", it can also used when referring to specific days or dates, in the future or in the past.
Finally, if you're looking ahead in a calendar, for example, and you know that on certain days the weather will be favorable, you could say
I don't know exactly when we'll go but we really must visit them one of those days.
you should use
One of THESE days
because these signifies upcoming events, that are in the future, while those signifies past events. for eg :
One of THESE days I'm going to the gym.
This signifies that I am planning to go to a gym in the upcoming days, whereas
THOSE were the days when I used to go to the gym.
This sentence states that I am remembering past events , like I remember going to the gym
so, saying that
I don't know exactly when we'll go, but we really must visit them one of these days.
means that you are planning to go visit someone in the upcoming days.
When you will do something at a future date you say "one of these days".
e.g. "One of these days Alice, Bam! Right to the moon!".
When you will refer to, typically a bad day, you say "one of those days".
e.g. "I tripped and fell into a mud puddle; today is one of those days."
Both can be used in a purely literal sense for example.
'Any one of these days would be suitable for a meeting: 2nd January, 24th January or 1st February.'
'I am definitely busy on one of those days but I will be free for at least one, I will let you know.'
However, in isolation both phrases do have specific additional meanings.
'One of those days', generally means a bad day. as in 'Sorry if I'm in a bad mood I've just had one of those days'. Here it is given addition meaning by inflection and/or context.
However if can be positive if qualified 'It was one of those days where everything falls into place'.
In both cases there is an implication that it is a state of affairs that both parties will be familiar with.
'One of these days', can be used to indicate an intention to do something or a belief that something will happen at some indeterminate time in the future, eg. :
'One of these days I will get around to fixing that leak'.
'He will get into serious trouble one of these days'.
In general terms 'these' implies something immediate or close to hand eg.
'Would you like to try any of these apples?'
Whereas 'those' implies something a bit more distant or removed from the speaker eg.
'Those hills are a nice place for a walk'.
Most of you seem lost if not completely incorrect. The usage is not idiomatic, and, contrary to what some said, there is nothing inherently negative about those.
First, these and those: These two words are plural demonstrative pronouns, and the meaning distinguishing the two has to do with proximity. These is used to refer to things relatively close; those is used to refer to things relatively distant. The context resolves the relativity of the two words. These words serve for both time and space.
Simply put, the use of those days, as opposed to these days (implicit or explicit), refers to a period that isn't now. It can be in either the past or the future.
For example, the song "Those Were the Days" (an old song from the 1920s, made a hit again by Mary Hopkin under Paul McCartney's production) refers to a good time in youth:
Those were the days my friend
We thought they'd never end
We'd sing and dance forever and a day
We'd live the life we choose
We'd fight and never lose
For we were young and sure to have our way.
—Those Were the Days.
As you can see, there is nothing inherently negative in those: it just mean days that are not now, not these days. You can also speak about those days that lie ahead:
Those days will be much brighter.
That, which is the singular form of those, is often paired with day to refer to the dies irae, judgment day or, literally, the day of anger. It comes up in Ingmar Bergman's The Seventh Seal in this exchange between the Knight (Max von Sydow) and Death (Bengt Ekerot):
But one day they will have to stand at that last moment of life and look towards the darkness.
When that day comes ...
—The Seventh Seal. (emphasis mine)
It is common in the literature that speaks of the day of reckoning to refer to it simply as that day because of fear of the day, rather like in the Harry Potter series Voldemort is more commonly referred to as He-Who-Must-Not-Be-Named.
So to the OP's question:
When should we use "one of these days" and "one of those days"?
It is a judgment call, but that judgment should depend upon whether the day that you are talking about belongs more to the period that you are in, or the period that you will be in (or were in), and the sense of proximity is drawn from the context of the conversation.
He was starving, had no money at all, so he looked forward to drawing Social Security because when those days arrive he will be able to eat a little something.
[In those days] We used to have plenty of food, but these days the pantry is bare.
My rickety old grandfather is a standup comedian. In those days when he used to tour constantly, we never saw him. But these days he keeps us laughing constantly.
My daddy used to say that one day America will have to face a terrible reckoning. Judging from the progress of the electoral campaigns, those days may be hard upon us.