how to say “the day before yesterday” in Hebrew
שִׁלְשׁוֹם
For example:
דִּבַּרְתִּי אִתּוֹ שִׁלְשׁוֹם.
I spoke with him the day before yesterday.
The modern usage of שלשום as the day before yesterday derives from the Biblical-Hebrew expression כִּתְמוֹל שִׁלְשׁוֹם 
listen and repeat – as of yore or as things used to be. Nobel Laureate Shmuel Yosef Agnon entitles one of his novels with this phrase.
listen and repeat – as of yore or as things used to be. Nobel Laureate Shmuel Yosef Agnon entitles one of his novels with this phrase.The word שלשום itself has שָׁלֹשׁ listen and repeat – three – as its root, since what occurred שלשום goes back three days if today is included in the count.
listen and repeat – three – as its root, since what occurred שלשום goes back three days if today is included in the count.