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a great question, KohenMatt.
the short answer is that they were all in 'unity' - like it states, They were all there with one accord.'
what Godly joy that all must have felt!
as it was commanded, as in the days of old, they didn't keep it according to the 'dictates of their own hearts'
like the Word says, 'let us all speak the same thing, for if we do not, there is confusion and babble and
every wind of doctrine will be blowing amongst the brethren and that rushing mighty wind spoken of in Acts,
becomes a breath of death.....
Another short answer (albeit perhaps uncomfortable to some) is that they were celebrating the Biblical Feast of Shavuot (Feast of Weeks.) They were gathered together because they were celebrating that Feast, and God saw fit to grant them this manifestation of His Spirit because they were celebrating that day.