"This is the day the Lord has made: let us rejoice and be glad in it".
Until He first appeared to them on Easter Day, after His passion and death, Jesus' disciples had hidden in fear behind locked doors. Now they know He's alive, though He 'disappears' in a strange way after each time they see Him. But they are 'free' now and confident enough to go fishing in Galilee with Peter, while they wait for¡K for what? At this stage they don't know yet. Would He 'appear' to them again? When, how, where? His final 'disappearance' into heaven, and the Pentecost event had yet to happen.
And then, on the shore of the Sea of Tiberias, one of the most intimate and personal gatherings of Jesus with those seven of His close friends who'd gone fishing. In a kind of parable of their coming Mission, it was He who guided their hands to find the shoal of big fish whose weight didn't tear their nets: He had a charcoal fire waiting, some bread and fish ready, and an invitation to come eat with Himself.
Soon enough, for Peter and John there would be another occasion behind locked doors, when they would be arrested for curing a crippled man and for talking to people about Jesus' resurrection and salvation in His name (today's first reading). There's no hiding now, no fear. Now they are the Lord's fishermen in the Lord's ocean. Peter was 'filled with the Holy Spirit' as he answered the authorities' questioning, clearly claiming he and John had acted in the power of Jesus' name.
"This is the work of the Lord, a marvel in our eyes"