What would it have been like when the apostles first celebrated the Eucharist?

It seems that Peter would be the one to lead. Can you imagine, though, going to your friends, “Hey guys, He said, ‘Do this,’ so why don’t we give it a go?” And then trying to figure out precisely what it is that you were to be doing, with everyone watching? Even if you remembered everything with perfect clarity, to ask yourself if this is what He meant? Would James and John be asking to be the celebrant for the next day?

I don’t think they would have celebrated Mass while the Risen Lord was with them, but what about the 10 days until Pentecost? Or did they have to wait until the coming of the Holy Spirit? I’d think it would have to be after Pentecost. Because 1) it’s the calling down of the Holy Spirit which transforms the gifts and 2) if they partook of the Eucharist, presumably they wouldn’t have been hiding scared in the Upper Room at Pentecost, but would have been able to draw upon His strength.

So, if they waited until Pentecost, what did they do on that Sunday in between?