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.
ell.stackexchange.com/users/9259/Khan
The short answer.
The two phrases are idioms.
one of these days
On some day in the future one of those days a day when everything goes wrong
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.
For example, During the last week of June, one of those days I will mail this letter.
Back in high school, I remember one of those days I have quite a terrific day.
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.
ell.stackexchange.com/users/36187/Em.
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.
ell.stackexchange.com/users/34417/Devanshi Parikh
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."
ell.stackexchange.com/users/36231/Bob Smith
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'.
ell.stackexchange.com/users/32210/Chris Johns
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 THOSE days" vs "one of THESE days"
ell.stackexchange.com
<phrase-usage><usage><phrase-choice><determiners><demonstrative>
ell.stackexchange.com/questions/93066
ell.stackexchange.com/users/34700/Nguyễn Quốc Việt
"One of those days", often means a bad day,  when everything goes wrong.  However, it can also be used when referring to specific days or dates, in the future or in the past.  "One of  these  days" can refers to upcoming events or plans in the future, as in "One of these days I'm going to the gym".   When you are talking about doing something at a future date you might say "one of these days". "One of these days" should be used, because it means "in the near future", "in the upcoming days", it signifies upcoming events. On the other hand, "One of those days" is used to signify events/days/dates that are in the past, and oftentimes it is used as a phrase to describe a bad day when everything goes wrong.
"One of these days" should be used, because it means "in the near future", "in the upcoming days", it signifies upcoming events. "One of those days" is used to describe a bad day when everything goes wrong. "One of those days" is used to signify events/days/dates that are in the past.