One of these days means at some unspecified future time. One of these days I will sort out that shed.
It is used for things people intend and never do, so the vagueness is doing real work. Said as a threat it means somebody will get what is coming to them.
1phraseone of these daysAt some unspecified future time.
“One of these days that gate is going to fall off.”
one of these days is a phrase.
Practised in Small Talk & Everyday Phrases