Thats not true...
Yesterday means "any time in the past".. though particularly used for the recent past.
Yestermonth is a word
Yesterweek is a word too
@LouisIngenthron the definition of yesterday as "any times in the past, but particularly recent past" is a modern definition
yestermonth is not a common enough word to be recognized by all dictionaries, so it is debatable.. but there is modern expert usage I was able to find at least as late as 2009 and dictionaries (some) recognize and use this usage as an example:
https://www.wordsense.eu/yestermonth/
@freemo Even the examples they give for the secondary definition that matches yours are archaic idioms.
If someone says "I dropped the check in the mail yesterday.", then no reasonable person would understand that to mean anything other than "the day before today."
@LouisIngenthron Good debate though, thanks.