Difference between 'those days' and 'these days'

[Sajjad Qamar]

Test No. incompl/inter-38 "Save the Day", question 9

I don't know exactly when we'll go but we really must visit them one of ......... days.

(a) this
(b) these
(c) that
(d) those

Test No. incompl/inter-38 "Save the Day", answer 9

I don't know exactly when we'll go but we really must visit them one of these days.

Correct answer: (b) these

Your answer was: incorrect
I don't know exactly when we'll go but we really must visit them one of those days.
_________________________

Please explain the meanig of "these days" .

[Alan]

This is an expression meaning some time in the future. Those days as in the expression: in those days we used to ... refers to a time in the past.
_________________
English as a Second Language
You can read my ESL story Present Simple

[Saneta]

Mr Alan, please tell me why is there ,,when we'll go'' and not: ,,when we go''?
Shouldn't it be always Present Simple tense after: ,,when''?
Thanks a lot!!!