The opening lines of the Canterbury Tales constitute a learned version of the "reverdi," a simple lyric celebrating the return of Spring after the harshness of winter, a common form of medieval French lyric. It became widespread in English as well. widespread in English as well. The most famous example in is the "Cuckoo song," which dates from the twelfth century:

Sumer is i-comen in.
Groweth seed and bloweth meed
And springth the wude nu.
Sing cuccu!

I suppose that little songs like this go back to earliest antiquity -- the reassuring return of vegetation and fertility, and of the sun -- especially in Northern Europe - - after the cold and dark winter.

The standard love lyric builds upon this return of spring song by adding human love. Spring brings a great outburst of energy in nature, the birds begin to sing again, and nature stirs its creatures to love:

Western wind, when wilt thou blow,
The small rain down can rain?
Christ, if my love were in my arms,
And I in my bed again!

When Spring arrives, love comes with it. Here is a typical opening of a lover's complaint:

When the nightingale singes
The wodes waxen grene,
Leaf and gras and blossom springes
In Averil, I wene,
And love is to min herte gon
With a spere so keen.

And on then into the story of his love.

These are all in the background of Chaucer's opening lines, echoing in the minds of his listeners. There is in the opening lines of the Canterbury tales a kind of celebration of fertility, the same joyful welcome to spring that we have in Sumer is icomen in, and it has all the elements of the conventional first stanza of the love lyric -- the singing bird, the springing flower, and the time -- April or May, early spring.

Chaucer's first surviving work was a translation of The Romance of the Rose, or a good part of it, into English. And that poem itself defined the high courtly style. If you were going to write a dream vision you observed the conventions as they were used in The Romance of the Rose, including the obligatory description of spring as it is set forth in the opening of the Roman de la rose. There the brief amatory reverdie which we saw in that litte poem "When the nightigale singes." is greatly elaborated.

That it was May thus dremed me
In time of love and jollite
That al thyng gynneth waxen gay
For there is neither busk nor hay
In May that it nyl shrouded ben,
And it with new leves wryen. cover
These greves eke recoveren grene,
That dry in wynter ben to sen,
And the erthe waxeth proude withal
For swete dewes that on it falle . . .

And the birds begin to sing:

To make noyse and syngen blythe
Than is blisful many sithe
The chelandre and popinjay
Then yonge folk entended ay
For to ben gay and amorous

It is the same movement we saw in When the Nightingale Sings -- the time is spring, the flowers bloom, the birds sing, and then young love. All these elements, even love, are in this opening sentence of the Canterbury Tales -- the Spring setting, the birds, the flowers, the impulse toward love, all elegantly elaborated in the style of The Romaunt of the Rose.

To this Chaucer adds another tradition of the celebration of Spring, that of the learned Latin tradition. Chaucer does not simply tell you that the dews fell on the earth as Guillaume does. He gives instead a brief scientific description, teling how the dews engender the virtues -- which means powers -- which are the humors that will produce the flowers. Here is how Spring was described in an actual scientific Treatise, Vincent of Beauvais' thirteenth-century encyclopedia of Natural History. It was a work that Chaucer knew very well and of which may even have -- I think probably -- owned a copy:

Indeed the sun penetrating to the roots of grasses and plants, draws out the freezing humor which winter had brought, and the grasses and plants, feeling their emptiness, draw in the humor of the earth, which adding to it the heat of its own humor, the hear of the sun transmits it to the plants, and thus they are revived and grow green; whence it is that this month is called April, since this is when the earth is opened -- aperitur. (Vincent of Beauvais, Spec. Nat. 15.66, De Vere).

Or, somewhat more poetically, but still scientifically, here is Bartholomaeus Anglicus, as translated by Chaucer's contemporary John Trevisa:

The pores of the earth are opened, and humors begin to move upwards in beasts, trees, and man . . . and therefore April is painted with a flower, for in that month April the earth hath that beginning to be clothed and adorned with flowers (modernized from John of Trevisa, pp. 530-31).

Chaucer not only says it is April, he defines the time by exact reference to the Cosmos -- to the young sun just emerging from the zodiacal sign of the Ram, so that the action takes place in a grand cosmic setting -- on earth, surrounded by the nine spheres -- the seven planets, the fixed stars, the primum mobile, all moving in a harmony produced by the reconciliation of opposites, the music of the spheres. Chaucer learned this from Latin literature, and when he wrote the opening lines of Troilus he may even been thinking directly of this passage from a work by Guido delle Colonne, the Historia Troiae, a work which Chaucer read and used in writing his own story of Troy, Troilus and Criseyde:

It was the time when the young son (maturans sol), making its course under the oblique circle of the zodiac was now entering the Ram . . . the equinox of the first spring was celebrated, when the time of gentle and serene air comes to mortals, than when Zephyrus brings the soft waters of rain, and when the moisture is drawn from the bosom of the earth up into the high branches of the trees, the leaves grow, the fields turn green . . . the birds sing and produce the sweetest harmonies, then when we are in the midst of April. (Freely, nay wildly translated)