We’re talking about a location in time, so “these” days refers to the present — our current location in time. “Those days” would usually refer to the past — distant from us in time.

This sentence only makes sense to me if someone is pointing at dates on a calendar while saying that sentence, in which case you would first say "these". It would sound better to also say "on one of these days", including the"on" which is typically used when talking about a specific day, like "on Wednesday" or "on January 1st" There is a second case where you may be able to say "those" if you first point to some dates, and then you point to some older dates immediately after. For example "you heard from me on one of these days (point to first week of this month), or on one of those days (point to one month ago)" but I think saying "these" both times since you're pointing instead of saying "those" the second time is more natural for American English. There's also a special case of "those days" if there was something special or memorable about the day, like everyone is always/consistently really mad on those days. Maybe it's a day when there's a lot of work to do. "You heard from me on one of those days", where it's a special type of day where something specific always happens.