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.