http://revach.net/article.php?id=298

Section: Moadim   Category: Shavuos
Shavuos: Will The Real Atzeres Please Stand Up?

The Yom Tov of Shavuos is also called Atzeres just like Shmini Atzeres. The Ramban explains that this is because Pesach and Shavuos are considered to be connected and they are one long Yom Tov.  The days between Pesach and Shavuos are considered like Chol ha'Mo'ed.  Matan Torah on Shavuos is the crown jewel of the entire Pesach just like Shmini Atzeres is the crowning day of Succos.  This also explains why Shavuos does not have a set date on the calendar, rather we relate to it as 50 days after Pesach.