Pentecost lasts 7 weeks or about 50 days. It is a celebration in the Christian faith where they celebrate the time when the Holy Spirit came to the Apostles or the followers of Jesus. This time reflects after the crucifixion and resurrection of Jesus. It happens after Easter. Easter is determined by the use of the full moon and the date, March 21st.
It may seem like this is a long period of time considering that many Christian holidays only last one day, but Pentecost lasts 50 days. It is very difficult to fully celebrate Pentecost every single day as Christians do for Easter and Christmas. That is why it may seem that many Christians do not celebrate Pentecost, but the church does.