Do you basically celebrate the major religious holidays twice, about two weeks apart?

Yes, Pentecost is significant because it's when the Holy Spirit empowered the disciples, enabling them to preach the Gospel and begin the Church's mission.

The text in Acts 2:1-2 mentions the disciples being "together in one place" when the Holy Spirit descended, but it doesn't specify the location as an upper room. Acts 2:41 states "They then that received his word were baptized, and there were added unto them in that day about three thousand souls." That's really amazing.

That is a beautiful story in the New Testament, about the Holy Spirit descending and about the disciples finding they can speak to people who are in Jerusalem as pilgrims from all over the known world, and be understood.
In Judaism we just celebrated Shavuot, which is historically the same as Pentecost, last week. It is nice when holidays overlap.

Peter's message was amazing, and the peoples response even more so! You might have expected the people to get angry and even stone the apostles but instead they wanted to know how to be saved!

I agree it is very inspiring