Another aspect of the deacon's service tot he Word of God is that of preaching, including preaching the homily during the Eucharist and at other sacramental and liturgical celebrations. The basis for this ministry is found in the Code of Canon Law (for the Latin Church; a similar canon exists in the Code of Canons for the Eastern Catholic Churches):
Can. 764 Without prejudice to the provisions of can. 765, priests and deacons, with the at least presumed consent of the rector of a church, have the faculty to preach everywhere, unless this faculty has been restricted or removed by the competent Ordinary, or unless particular law requires express permission.
Let's take a closer look at this canon and what it means.
1) Notice that this canon extends the faculty to BOTH priests and deacons. Unlike other canons which refer specifically to priests, and still others which apply specifically to deacons, this canon speaks to both orders. So, the legal basis by a which a priest (other than a pastor) as well as a deacon preaches at Mass flow from the same law.
2) Notice that the faculty extends "everywhere"; whether I'm in Washington, DC, Peoria, Illinois or Monterey, California or Rome, Italy, the law says I have the faculty to preach.
3) Significant, of course, is the subordinate clause about "with the at least presumed consent of the rector of a church". The PASTOR (not just any priest, but the priest who holds the office of pastor) may legitimately restrict the preaching of any priest or deacon within his pastoral jurisdiction. The law also says that the Ordinary (usually the diocesan bishop) may also restrict or remove this faculty. But notice that these restrictions must be made explicit; otherwise, the deacon (and priest not the pastor) HAVE the faculty. For example, let's say that the document conveying diaconal faculties (this document is called a "pagella") to the deacon does NOT include the faculty to preach. What does this absence mean? Given c. 764, it means that the deacon HAS the faculty to preach and that it is not restricted or has not been removed by the bishop. If the bishop does not want a particular deacon to preach, he has to put that in writing.
4) Furthermore, if the pastor of the place decides he doesn't want the deacon or another priest to preach within his jurisdiction for whatever reason, I would strongly recommend that this restriction be communicated to the bishop. The bishop (and in this case, the law itself) extends faculties to priests and deacons with the presumption that they will be exercised for the good of the People of God. If this is not going to be the case, then the bishop should be informed.
5) Liturgical law (contained in the General Instruction on the Roman Missal and in the other praenotanda to the various sacraments) also contains regulations regarding preaching. With regard to the deacon preaching at Mass, the GIRM notes that the deacon preaches "on occasion". The norm is for the priest-presiding to preach, but he may also ask the deacon to preach the homily. This has caused no little confusion: just what does "on occasion" mean?
Some pastors have interpreted this too mean "rarely" or "only in extraordinary circumstances." This is not correct. What the Holy See was trying to correct were abuses in some parts of the world (not so much in the US) in which priests were never preaching, but turning over that responsibility to others (deacons or lay people). However, given the law we've already examined, the deacon IS an "ordinary" preacher and, while we don't preach at EVERY Mass, we should certainly preach with certain regularity.
Finally, and perhaps most important of all, diaconal preaching should have its own unique character. Deacons can bring their own experience of family and professional life to bear in their homilies. Just as priests can offer insights from the perspective of their own experience, so too can deacons. This goes much deeper than simply telling parishioners what he and his family did on vacation! Rather, the deacon can offer insights about the practical challenges of living out the Word of God in school, on the job and in the family. He can also highlight what he has experienced in his own ministries of charity, and the very real needs of the people he is serving in prisons, hospitals, on the streets and in soup kitchens. This desire to encourage uniquely diaconal preaching is reaching new maturity. Just over the last two years, I have assisted three graduate students completing doctorates in ministry who wrote about this very topic. Two were written by deacons and one by a priest.