"Good King Wenceslas" is a Christmas carol that tells a story of a Bohemian king going on a journey and braving harsh winter weather to give alms to a poor peasant on the Feast of Stephen (December 26, the Second Day of Christmas). During the journey, his page is about to give up the struggle against the cold weather, but is enabled to continue by following the king's footprints, step for step, through the deep snow. The legend is based on the life of the historical Saint Wenceslaus I, Duke of Bohemia or Svaty Vaclav in Czech (907-935). The name Wenceslas is a Latinised version of the modern Czech language "Vaclav".
Tempus adest floridum
The tune is that of "Tempus adest floridum" ("It is time for flowering"), a 13th-century spring carol in 76 76 Doubled Trochaic metre first published in the Finnish song book Piae Cantiones in 1582. Piae Cantiones is a collection of seventy-four songs compiled by Jaakko Suomalainen, the Protestant headmaster of Turku Cathedral School, and published by Theodoric Petri, a young Catholic printer. The book is a unique document of European songs intended not only for use in church, but also schools, thus making the collection a unique record of the late medieval period.  A text beginning substantially the same as the 1582 "Piae" version is also found in the German manuscript collection Carmina Burana as CB 142, where it is substantially more carnal; CB 142 has clerics and virgins playing the "game of Venus" (goddess of love) in the meadows, while in the Piae version they are praising the Lord from the bottom of their hearts.  The text of Neale's carol bears no relationship to the words of "Tempus Adest Floridum". In or around 1853, G. J. R. Gordon, the British envoy and minister in Stockholm, gave a rare copy of the 1582 edition of Piae Cantiones to Neale, who was Warden of Sackville College, East Grinstead, Sussex and to the Reverend Thomas Helmore (Vice-Principal of St. Mark's College, Chelsea). The book was entirely unknown in England at that time. Neale translated some of the carols and hymns, and in 1853, he and Helmore published twelve carols in Carols for Christmas-tide (with music from Piae Cantiones). In 1854, they published a dozen more in Carols for Easter-tide and it was in these collections that Neale's original hymn was first published.  The tune has also been used for the Christmas hymn Mary Gently Laid Her Child, by Joseph S. Cook (1859-1933); GIA's hymnal, Worship uses "Tempus Adest Floridum" only for Cook's hymn.

Q: what is temps adept floridum?
A: The tune is that of "Tempus adest floridum" ("It is time for flowering"),
what is special about the tune?
a 13th-century spring carol in 76 76 Doubled Trochaic metre first published in the Finnish song book Piae Cantiones in 1582.
a 13th-century spring carol in 76 76 Doubled Trochaic metre first published in the Finnish song book Piae Cantiones in 1582.