From the Great Kedusha Comes Teshuva

Similarly, the verse states (Vayikroh 23:15): ‘U’Sefartem lachem mimacharas haShabbos. (And you shall count for yourselves from the day after Shabbos).’ The Avodas Yisroel explains the word ‘u’sfartem’ to mean a ‘sapir’ (diamond). Thus the pasuk is telling us to make ourselves illuminated by doing teshuva for all of our sins. We are told to do this ‘from the day after Shabbos’, which is a reference to the first day of Pesach. We are told to take the holiness of this day and allow it to leave an imprint of light upon us that will last for the coming weeks.