This is the 614th most frequent Hebrew word.
שלשום
The day before yesterday.
In this sentence, 'שלשום' indicates an event that occurred two days before the current day, meaning the day before yesterday.
הייתי בירושלים שלשום.
I was in Jerusalem the day before yesterday.
Here, 'שלשום' places the timing of the conversation with my friends as specifically two days ago.
חברים שלי סיפרו לי שלשום על התוכנית החדשה.
My friends told me the day before yesterday about the new plan.
This use of 'שלשום' specifies the time of the short rainfall as being two days ago.