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/
@LouisIngenthron @freemo Related fun words:
The day before yesterday is ereyesterday.
The day after tomorrow is overmorrow.
Yesterday evening is yestreen.