This Sunday is the Day of Pentecost. The day where, in Acts 2, the Holy Spirit "fell" upon the gathered ekklesia, with tongues of fire appearing over their heads. It was the original occurance of speaking in tongues.
Now the early believers normally met on Saturday evenings (as in Acts 20:7-12), which marked the end of the Sabbath (to the Jewish mind "the first day of the week" began Saturday evening at sundown, not Sunday morning as we think, who have been trained by Roman custom to count days differently than God established). On this day they were meeting Sunday morning, because it was the Feast of Weeks, or Shavuot; pentecost (fifty) in Greek. In Leviticus 23:15-22, the ekklesia is charged with holding a holy convocation, or assembly, on that day, which is what the believers were doing. It was one of the seven annual feast days of the Lord, and one of the three high holy feast days, in which males were to appear before the Lord at the Temple in Jerusalem. It was also one of the three harvest feast days.
It was on this day, in history, that the Ten Commandments were given to Moses inscribed on tablets of stone with the finger of God. The rabbis even have the tradition that the seventy elders of Israel on this day spoke, by the power of the Holy Spirit, in the seventy tongues of the nations, as a prefiguring of all the nations that would one day worship and serve the Lord in spirit and in truth. The outpouring of the Holy Spirit on this day fulfills Jeremiah 31:31-34:
"Behold, the days are coming, declares the LORD, when I will make a new covenant with the house of Israel and the house of Judah, not like the covenant that I made with their fathers on the day when I took them by the hand to bring them out of the land of Egypt, my covenant that they broke, though I was their husband, declares the LORD. But this is the covenant that I will make with the house of Israel after those days, declares the LORD: I will put my law within them, and I will write it on their hearts. And I will be their God, and they shall be my people. And no longer shall each one teach his neighbor and each his brother, saying, ‘Know the LORD,’ for they shall all know me, from the least of them to the greatest, declares the LORD. For I will forgive their iniquity, and I will remember their sin no more."
Shavuot/ Pentecost is a memorial feast, it is the memorial of the day when, by the infilling of the Holy Spirit, God inscribed His Law on the tablets of our hearts. It just so happens that it is also Memorial Day weekend. The death that I will be remembering this weekend is the death of my old man, who was buried with Christ, and I will spend Shavuot giving thanks to God for creating in me a new man, created after the likeness of my Lord and Savior, Jesus Christ.