Why do we celebrate Easter only 2 days after Good Friday? I have always wondered why we celebrate Easter only two 24 hour periods since Good Friday. Throughout Scripture we see the Apostles and Paul talking about Jesus's Resurrection was three days after his crucifixion according to the Scriptures (1 Corinthians 15:4). I feel like I read somewhere that a day back in the 1st Century meant something different than what we believe in the 21st Century. This should be something that we are taught when we are young instead of me asking when I turn 17 in 2 months.
3 days and 3 nights

To an American with a clock, this phrase automatically makes us think of 3 entire days and nights or 3-24HR periods.  This phrase to a Jew in the 1st century and before would have meant to the 3rd day or on the 3rd day.  

Look how Jesus interchanges the language: 


  Matt 16:21 From that time Jesus began to show His disciples that He
  must go to Jerusalem, and suffer many things from the elders and chief
  priests and scribes, and be killed, and be raised up on the third
  day.
  Matt 12:40 for just as Jonah was three days and three nights
  in the belly of the sea monster, so will the Son of Man be three days
  and three nights in the heart of the earth.


We also have an OT reference that lets us know how a Jewish person would interpret this phrase at that period of time.  


  Esther 4:15 Then Esther told them to reply to Mordecai, 16 “Go,
  assemble all the Jews who are found in Susa, and fast for me; do not
  eat or drink for three days, night or day. I and my maidens also will
  fast in the same way. And thus I will go in to the king, which is
  not according to the law; and if I perish, I perish.”
  
  Esther 5:1- 5 Now it came about on the third day that Esther put on
  her royal robes and stood in the inner court of the king’s palace in
  front of the king’s rooms, and the king was sitting on his royal
  throne in the throne room, opposite the entrance to the palace. 2 When
  the king saw Esther the queen standing in the court, she obtained
  favor in his sight; and the king extended to Esther the golden scepter
  which was in his hand. So Esther came near and touched the top of the
  scepter. 3 Then the king said to her, “What is troubling you, Queen
  Esther? And what is your request? Even to half of the kingdom it shall
  be given to you.” 4 Esther said, “If it pleases the king, may the king
  and Haman come this day to the banquet that I have prepared for
  him.” 5 Then the king said, “Bring Haman quickly that we may do as
  Esther desires.” So the king and Haman came to the banquet which
  Esther had prepared. 6 As they drank their wine at the banquet,
  the king said to Esther, “What is your petition, for it shall be
  granted to you. And what is your request?