What are the day six stories all about in The Decameron? What message is Giovanni Boccaccio sending?

The Florentine chosen to set the topic of stories on day six is Elissa, who determines that they will tell tales of characters who use clever rhetoric to avoid sticky situations or to succeed. As with the other ten days, there are ten stories told on day six, too many to address in detail here.  But a look at one of the most famous, “Friar Onion,” might demonstrate quite well the message Giovanni Boccaccio is suggesting to readers.  Although it is definitely a tale of satire on the church, like several other stories of The Decameron, it also adheres to the day six theme of characters using their intelligence and witty rhetoric to escape embarrassing situations.   
Friar Onion, a traveling monk/preacher who has ingratiated himself amongst the naive parishioners of the little hamlet of Certaldo, has intrigued his audience with promises of a glimpse of a holy relic--a feather from the Angel Gabriel.  He intends to preach a story designed to trick the villagers into giving him (the church) more money than they can probably afford to give.  When he pulls out the relic, he realizes that several of his fellow monks have played a trick on him, replacing his parrot feather with a lump of coal. Hiding it quickly, he immediately launches into a colorful story designed to mislead the unsophisticated crowd.  He convinces them that in a miracle, the feather has been replaced with “the coals on which the blessed martyr Saint Lawrence was roasted.”  And it just so happens to be the feast of Saint Lawrence in two days.  In fact, for a little extra money, each parishioner can receive the mark of the cross with the coal as a protective blessing. In the end, Friar Onion’s quick wit and skill with language allow him to not only escape the embarrassing trick but to actually fill the coffers even more than he would have with his original sermon.
The satire on the church here is clear, but in keeping with the other “day six” tales, it also celebrates human intelligence.  Boccaccio suggests that those who apply their quick wit and skills with rhetoric (whether moral or not) simply gain more out of life, while those who fail to think for themselves, like the villagers in “Friar Onion,” often have no one to blame for their folly but themselves.
http://sites.fas.harvard.edu/~chaucer/special/authors/boccaccio/boc-6-10.html