's fool or clown, sings the "Carpe diem" song. Despite being a "fool," Feste, like King Lear
's fool, is a sharp observer of those around him who, as a witness to the main action, comments wisely. In this comedy about love, he notes more than once the fleeting nature of love, youth and beauty. "Carpe diem" means "seize the day" and is usually used in the context of love: this genre of poetry advises lovers not to wait around, because none of us know how long we have. That is true in any age, but in Shakespeare's day, without modern medicine, all the more so. And in any age, youth ends all too soon.
The theme of this song is love and its connection to youth: the clown sings that love is for the present day. Lovers will not be young forever, for they will not remain "sweet and twenty." Now's the magic moment, so seize it, the (wise) fool advises.
The carpe diem theme emerges most clearly in the second of the song's two stanzas: the fool says love is in the present, not the hereafter, another common theme of carpe diem (Marvell's "To his Coy Mistress," for example, has a stanza envisioning the lover's body rotting in the tomb to encourage her to seize the moment and have an affair with him). Here the fool says that if you are happy today, laugh today, because nobody knows what tomorrow will bring. Delay will not bring abundance of love but simply use up the short time love has. Finally, the fool advises kissing the lover, for youth's season won't last:
O Mistress mine, where are you roaming?
O stay and hear! your true-love's coming
That can sing both high and low;
Trip no further, pretty sweeting,
Journeys end in lovers' meeting—
Every wise man's son doth know.
What is love? 'Tis not hereafter.
Present mirth hath present laughter.
What’s to come is still unsure.
In delay there lies no plenty.
Then come kiss me, sweet and twenty.
Youth’s a stuff will not endure.