Cosimo de' Medici the Elder commissioned this artwork at a time when Fra Angelico was established as a true Renaissance master. The Medici family itself is remembered as one of the most generous donors within all of art history, with multiple generations investing some of their wealth into the arts.
It is unclear whether the work took five years of work or is simply dated and being produced somewhere in this timeframe. The latter is far more likely although it is possible that the artist and his studio was working on multiple paintings at the same time and dipped in and out at various points. There is also the added variable of political interference which did delayed the work of some other Renaissance artists, most commonly within the field of architecture, such as Filippo Brunelleschi.
The San Marco Museum in Florence, Italy holds the finest collection of Fra Angelico paintings, by a distance and you will also find this altarpiece in their collection. Quite simply, anyone who appreciates his work will have to visit this venue and is fortunate in being able to see so much of his work under one roof, a rarity within the Renaissance. Those with specific paintings that they want to see should check ahead to see if it is on display as European galleries frequently loan out pieces for exhibitions.