Which One Is Correct?
What is the correct choice, 'one of these days' or 'one of those days'? Are they both correct but mean the same thing? Or Do they have different meanings?
Those Days or These Days
Showing Physical Distance
'These' and 'Those' are plural forms of 'this' and 'that'. They are called demonstratives. The main job of a demonstrative is to show us the 'proximity' of a noun to the speaker, i.e. the 'nearness' or 'farness' of an object or a person.
Showing Temporal Distance
Additionally, demonstratives can also show us 'temporal distance'. By using demonstratives, we can point to a time period 'near' to the present or a time period that's 'far' from now (in the past or in the future).
Here, 'these' shows that the speaker is talking about the days they are currently experiencing.
Here, 'those' shows that the speaker is talking about the days in the past. He is now reminiscing about them.
'One of Those Days' or 'One of These Days'
You might have heard the expression of 'one of those days' or 'one of these days'. Here, 'these' and 'those' do not necessarily mean a time in the past, present or future. They both have idiomatic meanings. Look at these examples:
'One of those days' mean 'a particular day when a lot of things go wrong'. It basically means 'a very bad day'.
'One of these days' means 'at some time in the future', we don't know when, it can be in two days or it can be in two years.