The quickest way to understand the parable of the great feast in the First Reading is to first read Luke's version of the same story in Luke 14:16-24. Most students of the parables are convinced that, in this case, Luke gives us the version closest to Jesus' life setting and that Matthew has given the parable quite a different turn to apply it to the pastoral needs of the community for whom he was writing.
So read Luke 14:16-24 first. Notice that in his setting, a dinner party, Jesus tells the story in response to a guest's dreamy statement: “Blessed is the one who will dine in the kingdom of God.” Jesus proceeds to tell of a banquet to which none of the invited guests come, all of them begging off with phony excuses. The would-be host then instructs his servant to invite the poor, crippled, blind, and lame. Finding that there is still room, he has the servant call people in from the highways and hedgerows. In Luke’s setting, the parable is a commentary on Jesus' ministry and who is responding, or not. To his dreamy dining companion Jesus is saying something that might be paraphrased as follows:
Buddy, the kingdom of God is not something in the distant future. In my own preaching and table fellowship I've been inviting people to the table of the kingdom of God regularly, but most of those invited reject me, coming up with excuses like, “He eats with tax collectors and sinners,” or “He heals on the Sabbath.” Meanwhile, it is the poor and those in need of healing who respond to my invitation to the reign of God. And even outsiders (Gentiles) will be included.
Thus far Luke.
By this time, the original readers would have caught the fact that Matthew has turned the parable into a mini-history of Israel, alluding first to the people's rejection of the prophets—both Hebrew prophets and Christian—then alluding to the (recent?) 70 CE destruction of Jerusalem by the Romans, interpreted as an act of God. The final scene, the king coming in to meet the guests gathered from the roads, is a “fast-forward” to the final judgment. If we are puzzled by the king's singling out a guest caught without a wedding garment, we miss the cultural note that the host of such a wedding feast would supply a closetful of festive garments for the guests, and it is this man's fault that he has not cooperated (none of the other “street people” seem to be lacking proper attire).What was, in Luke, an illumination of Jesus' kingdom preaching ministry becomes, in Matthew's hands, a cautionary tale. As in the parables of the Weeds and the Wheat and the Net (both in Matthew 13), or the Wise and Foolish Bridesmaids and the Sheep and the Goats in Matthew 25, this is a parable about divine judgment aimed at a complacent community. Having one's wedding garment on is a symbol parallel to having your oil-lamp ready for the advent of the Bridegroom. It means having fed the hungry, having clothed the naked, having housed the homeless and so on, as in the vision of the sheep and the goats (Mt 25:31-46), where Jesus gives away the secret.