Our parsha (Torah portion) devotes individual paragraphs for each chag (holiday), but when it comes to Shavuot (the Festival of Weeks), instead of focusing on the one-day chag on the 6th day of the Hebrew month of Sivan, the Torah describes a chag that starts, "after the Shabbat," (after the first day of Pesach (Passover)) and continues and develops for 49 days.
Is Shavuot actually a 7-week -long chag? Why is it defined by being "after the Shabbat," and not by its date?