In German* it depends on the tense of the verb. “On Monday I went to…” refers to the most recent Monday in the past. “On Monday I go to…” refers to the closest Monday in the future. Same for “This Monday…”. “Next Monday” refers to the second closest Monday in the future.
Just "Monday" is the upcoming Monday ("på mandag") and "next Monday" is the one after that ("næste mandag").
Everything else just seems needlessly confusing.