You shall count for yourselves from the day after the Shabbat, from the day that you brought the Omer of the waving; seven complete weeks shall they be. (Lev 23:15)
This verse discusses the laws concerning Sefirat HaOmer - the counting of the Omer which takes place between Passover and Shavuot. Rashi explains that ‘from the day after the Shabbat’ refers to the day after the festival, i.e. the second day of Passover. He further explains that the word ‘complete’ teaches us that one begins to count from the evening (the second night of Passover) or else the weeks are not truly complete. The word ‘u’sefartem - and you shall count’ is from the same root as the words ‘sapphire’ and ‘bright’ as if to say, ‘Work on ‘yourselves’ until you are shiny and bright.’ (The Maggid of Mezritch)
In counting sefira, the days between Pesach and Shavuot, we learn about the importance of time. Counting the days before Shavuot, the day on which we received the Torah, is a preparation for receiving the Torah, which emphasizes the value of time. Every Jew is obligated to occupy as much time as possible in Torah study, and not waste time that could be used for this purpose. (Likutei Sichot)