I was running into the same issue with not being able to disable the relative days, My issue was that when something was set to “The day after tomorrow” it made the module far to wide and overwrote other modules(Im using an older low res screen). To fix this I edited the file at MagicMirror/modules/default/calendar/calendar.js and replaced line 237 with the entry below, I changed it to two days which is much shorter but you can change it to whatever you want.
timeWrapper.innerHTML = this.capFirst(this.translate(“Two Days”));
The line originally was.
timeWrapper.innerHTML = this.capFirst(this.translate(“DAYAFTERTOMORROW”));