והיה כי תבוא אל הארץ (כי תבוא כו:א)

The words “כי תבא” are the acronym for כולם יעשו תשובה בחודש אלול – the main teshuva during the month of Elul is to prepare oneself to be able to daven the tefillos of the Yomim Noraim with pure kavanah. Therefore the Torah alluded to this specifically in the beginning with the parsha of ביכורים since tefilla is oftentimes called ביכורים, since it’s the first task of the day, as Chazal say: “One can’t start his daily tasks before davening.”