This is one parable you won't find in your child's Sunday school curriculum.
It is intended for theologically mature audiences only. In fact, without proper attention to the narrative context, I'm not sure this parable is conducive to a Christian sermon at all.
The narrative context reveals that Jesus directs the parable to the chief priests and elders of Jerusalem (Matthew 21:23), the very people who will arrest Jesus and hand him over for execution (the Pharisees are also mentioned at Matthew 21:45). The preceding parable (Matthew 21:33-46) has already gone some way toward explaining their opposition to Jesus. Recall that the tenants (chief priests and elders) appointed by the landowner (God) to oversee his vineyard (Israel) attempted to usurp the vineyard itself, going even so far as to kill the landowner's servants (the prophets) and, eventually, his own son (Jesus). The story ended, however, with the vindication of the son, the destruction of the tenants, and the handing over of the vineyard "to a nation that produces the fruits of the kingdom" (Matthew 21:43).
Thus last week's parable served to explain how the authority of Israel's traditional leaders was no longer valid in light of their rejection of Jesus. With their call to serve God poisoned by a sense of entitlement, they can no longer discern God's will, even when it is presented by God's own Son. Because this week's parable is also directed "to them" (autois, v. 1), we must not make the mistake of reading into it a judgment against the nation of Israel a whole. As with all the Gospels, much of Israel has in fact been drawn to the Jesus. Here Jesus simply continues the indictment of his opponents, but now through the analogy of a wedding banquet.
For an ancient society predicated upon honor and shame, nothing could bestow more honor (to oneself and, by extension, to one's family) than attending a royal wedding, particularly the wedding of the king's own son. This is the kind of event for which you make room in your calendar. Circle the date. Don't forget. Be there at all costs.
Conflict quickly surfaces; and it surfaces in two stages. The first stage is a simple summary: the king summons his guests via his servants, "but they would not come." He's snubbed by everyone. At this point the king already faces a tremendous amount of shame that, especially by ancient standards, must be remedied. In other words, he must find a way to save face. The disrespectful invitees face the likelihood of reciprocation or, depending on what kind of king this is, retribution.
Surprisingly, however, the king graciously extends a second summons, and with the specific instructions that his servants build up the event: 'Tell those who have been invited: Look, I have prepared my dinner, my oxen and my fat calves have been slaughtered, and everything is ready; come to the wedding banquet." In other words, you don't know what you're missing. Please reconsider. While this isn't a literal begging on one's knees, it might be the royal equivalent. The king really wants these people at the party.
Then the conflict turns violent. Some invitees only "neglect" (amelesantes, v. 5) the king's hospitality in favor of their own everyday concerns, "one to his farm, another to his business." The rest, however, mistreat and kill the royal servants, thus eliciting the wrath of the king. His forbearance now expired; the king slaughters "those murderers" (now deemed "unworthy") and fills his hall with whoever can be brought in off the streets, "both good and bad."
Who are these people from the streets? There are two possible referents. Because they are presumably common people and numerous enough to fill the king's hall, they could represent the many people targeted by Jesus' ministry: "the lost sheep of the house of Israel" (Matthew 10:6; 15:24). However, because the parable continues Jesus' indictment against Israel's leaders, they could also represent new leaders appointed by God, that is, the leaders of the church. This would be consistent with Matthew's interest in the apostolic/Petrine foundation of the church (Matthew 10:2-4; 14:28-29; 16:16-19; 18:18, 21-22; 19:28) and with the parable's subtle reference to the apostle Judas, the supposed "friend" (etaire, v. 12; see also Matthew 26:50) who proves to be a wrongly dressed infiltrator. Of course Matthew may not have envisioned a single referent, so these possibilities are not necessarily mutually exclusive.
Preachers will not be able to extract a "practical" lesson from this text. There are no nuggets of wisdom to be "applied" to a congregation. This is simply an unsettling parable of disobedience and divine retribution that seeks to explain an equally unsettling reality in the larger scriptural story, namely the rejection of Jesus by some (though hardly all) of his fellow Israelites (contrast the non-violent parallel of Luke 14:15-25, which targets a different audience and lacks the specific polemical thrust). The one heartening aspect of the parable is the analogy of the wedding banquet--the church is one big celebration of Jesus, with God as the host. But, one mustn't twist the parable into a sermon of assurance that simply overlooks the theme of judgment. Any parishioner who actually listens to the text will be rightly suspicious of this.
At the same time, a sermon focused solely on God's judgment upon Israel's late first-century leadership will not prove particularly edifying (or challenging) to a twenty-first century congregation. Explaining the parable in these historical terms, however, can help parishioners see more clearly the important starting point of Matthew's ecclesiology: the church is an extension of, and renewal of, the people of Israel. This is no minor clarification given that the parable, when extracted from its narrative and historical context, can easily be misread as supercessionist (the church replaces Israel). God's faithfulness to Israel is central to Matthew's Gospel, and the celebration of the Son marks the fullest expression of that faithfulness. Likewise the church is not a divine "do-over" but the very locus of that celebration.
While one cannot overlook the theme of judgment, then, one also cannot overlook the fact that the wedding banquet does occur. The king does not let a minor rebellion interfere with his love for the Son and his hospitality toward his subjects. If parishioners leave church understanding the logic of Matthew's retributive tone, while seeing themselves as the undeserving objects of God's hospitality toward Israel, then the sermon will have done justice to the parable.