Why Popes Wear White
A Piedmontese Dominican, Michele Ghislieri taught theology, defending church teaching to such effect that he came to the attention of the office then known as the Inquisition, which brought him aboard first as a regional supervisor, then calling him to its Roman hub. Depending on the pontificate, his zeal won him reward and scorn, each in their turn.
In 1566, he was elected to the chair of Peter. According to one account, "he began his pontificate by giving large alms to the poor, instead of distributing his bounty at haphazard like his predecessors." Pius worked to simplify the papal court, curbed other excesses of what had been a decadent period for the church's top ranks, and instituted the feast of the Most Holy Rosary, having reigned during the European victory at Lepanto which the 7 October observance commemorates.
Oh, and he "forbade bull fights," to boot.
But, of course, his most significant accomplishment was the tying up of the loose ends of Trent and the promulgation of the Missale Romanum of 1570 which, with few changes, served for almost four centuries as the liturgy of the Latin church. Its wider comeback was anticipated by many this morning, which had been among the dates tipped for the release of Pope Benedict XVI's long-expected motu proprio granting wider permission for celebration of the Mass that bears Pius' name.
The document didn't drop today, but reported elements of it have trickled into light over recent weeks.
Having been through at least four drafts, the impending release of the document -- a source of tension among much of the current pontiff's team -- has been confirmed on-record by two senior curialists, including the Secretary of State Cardinal Tarcisio Bertone. Its exact formulation remains unclear, but it's believed that the text will permit unfettered celebration (i.e. beyond the realm of episcopal sanction) of the Old Mass in private... which liturgies the faithful could just happen to hear about and show up for, albeit within a certain numerical limit. It's also likely to be stated that, in parishes, public celebrations according to the 1962 Missal cannot overtake the scheduled celebrations of the post-Conciliar Novus Ordo, the Mass promulgated by Pope Paul VI in 1969.
(Given the opposition of many bishops to the move, don't be surprised if a new fervor for enforcing the rules on bination and trination starts to pop up in certain places.)
Pius V died on 1 May 1572. He was beatified in 1672 and canonized in 1712. As a tribute to the sanctity of the Dominican pontiff, who refused to shed his community's habit on his election to the papacy, his successors have donned the now-famous white cassock ever since.
On the liturgical calendar established under his watch, his feast is celebrated on 5 May.