Graduations are full of mixed signals. You get to graduate, because you've finished what you were doing. But the official recognition of finishing is called "commencement," which means to start. Somehow, it's the end and the beginning. At the same time.
Pentecost ends the Easter season. While it has its own meaning, that meaning is also part of something bigger that includes the Resurrection and the Ascension.
The Resurrection and the Ascension complete much of what Easter started. Everything that Jesus told the Apostles would happen with his death comes into focus. But it's also clear that the Ascension isn't the last word.
Pentecost Sunday is, at least as far as Easter is concerned. Now we know what Jesus meant about receiving the Holy Spirit. But it's also the start of something that's never been seen before.
Pentecost is the end and it's the beginning.
Readings for Pentecost: http://www.usccb.org/bible/readings/052415-pentecost-day.cfm