The Torah commands us to count the days from Pesach to Shavuos in order to connect the Exodus on Pesach with the Giving of the Torah on Shavuos. The days of S’firas HaOmer are a time to rectify character attributes. Rav Moshe Feinstein explains that the name Shavuos, literally Weeks, so beautifully conveys this idea, since the holiday of Shavuos is an outcome of the weeks that precede it. For this very reason, the date of the holiday of Shavuos is determined by the 49 days of S’firas HaOmer, and not the date, because the 49 days prior, which are used to prepare, are essential to the holiday, and are required in full (see Shem MiShmuel, BaMidbar 5675).