Shavuot:

The Day the Torah was given.  This is the celebration of the monumental encounter between G-d and the Yisraelites at Mount Sinai, an event which changed mankind forever. Pentecost, as it also called, not only memorializes the first giving of the Law written on tablets of stone, but it also memorializes, on the same day many years later, the giving of the Ruach HaKodesh (Holy Spirit) as written in Acts Chapter 2.  The Law of G-d is written in the heart of the believer, as it states in Jeremiah 31:33.