The one dominant theme underlying both the plot and subplot of Twelfth Night can be gleaned from the title of the play and more importantly the subtitle, Twelfth Night; or, What You Will. To understand the meaning of the title and how it connects to the dominant theme, one must first understand the traditions surrounding the Twelfth Night holiday, other wise called The Feast of Epiphany. Epiphany fell on the twelfth night after Christmas day, which marked the day that the Magi, or three Wise Men brought their gifts to baby Jesus. While one might expect an Epiphany celebration to reflect on Christmas traditions, in Elizabethan England, it was actually apparently a "time when excess and license were expected to run rampant" (eNotes, "Twelfth Night: Themes"). Shakespeare even wrote the play for an Epiphany party being held at one of the Inns of Court in 1602, and that particular party was apparently known to be "absolutely secular and even quite bawdy" ("Shakespeare's Twelfth Night"). Epiphany celebrations, especially the one this play was written for, were times of "masques, revels, defiance of authority, and general foolishness" ("Shakespeare's Twelfth Night"). Epiphany was also a "time of merry-making, of hard drinking, and of romantic (or lusty) pursuits" ("Themes"). Shakespeare's play perfectly illustrates the nature of the holiday, as well as human nature as exhibited during the holiday, which brings us to his subtitle, What You Will, meaning "take what you will [from this play]." On the surface, Shakespeare's play seems to be a nice comic love story, but on a deeper level, it is a portrayal of the foolishness of human nature, especially human nature displayed on wild, lawless nights. Hence, the central underlying theme in Twelfth Night is the foolishness of man.
We see foolishness exhibited in Duke Orsino's obsession with Olivia; in Olivia's obsessive, prolonged grief over her brother's death; in Maria's plan to humiliate Malvolio; in Sir Toby and Sir Andrew's drunken, bawdy behavior; and even in Malvolio's self-righteous behavior and arrogance. The only character who does not exhibit foolish behavior is ironically Feste the fool, or court jester. Feste is the only character who makes insightful comments about human nature, such as pointing out that Olivia is truly the fool for grieving so severely over the death of a brother whose soul is in heaven, as we see in his lines, "The more fool, madonna, to mourn for your brother's soul being in heaven. Take a way the fool, gentlemen" (I.v.64-65). He even later insightfully calls Duke Orsino fickle, which indeed proves to be true.