Wow, am I behind! My apologies.
I was particularly partial to this mystery when I was a full-blown charismatic (it is, after all, one of the foundational Scriptural references of the movement).
My earliest memory of this mystery was learning about it in school in 6th grade. At that point, our teacher explained the tongues spoken by the apostles as spontaneously speaking a different language they'd never learned (modern example: as if Paul spoke French, and Andrew spoke Russian, and Bartholomew spoke Swahili...). I didn't like that very much. I had always preferred (and still do) to think of it as the apostles speaking the same language they'd always spoken, but the hearers in the crowd each hearing them in their native tongue (so it's not like you had to find the apostle speaking your language in order to get the awesomeness).
The massive conversions that happened that day literally blow my mind. Like, I actually can't wrap my mind around those numbers.
What I can focus on, though, is that this was really the first novena. The apostles, Mary, and possibly a few others had just holed themselves up in prayer (admittedly, perhaps also partly in fear) for nine days. (This is why it makes me cry when Ascension Thursday is moved to Sunday. Huzzah for Jersey bishops who've done this right all my life!) I try to always do a novena to the Holy Spirit before Pentecost.
And no "Happy Birthday, Church" celebrations on Pentecost. We'll talk about that at the fifth sorrowful mystery, when she was really born.