It’s obviously a love story; and, as for Night and Day, perhaps most importantly Day is an obstacle to love. As such, Wagner gives it a flexible little four-note motif, often called the “Day” motif:
…which we hear a thousand times over the course of the opera. These four notes open Act Two, played prominently by the full orchesta:
Curiously, Wagner used this musical figure over and over again, throughout his career, whenever there’s an obstacle to true love. In Lohengrin, these four notes are the core of the “Forbidden Question” motif, the bizarre demand Lohengrin makes of Elsa which dooms their relationship even before it begins. In Die Walküre, Fricka uses this motif, furiously, as she objects to the love between Siegmund and Sieglinde (and between Wotan and any number of his sexual partners). In Götterdämmerung, this motif characterizes the pretty Gutrune--who becomes an obstacle between those destined true lovers Siegfried and Brünnhilde. The motif is so simple--really, more of a musical tic than a legitimate motif--that Wagner can use it for all these different characters in all these different operas and make it sound appropriate in each case.
In Tristan, this motif is given the job of waving the proud flag of glorious Day. Night may be preferable to Day, for Tristan and Isolde, but Day has to have some kind of appeal. In Acts One and Two of Tristan, Day’s appeal is that of glory, honor, nobility, being a straightforward good guy, a knight in shining armor. You hear the Day motif, in this mode, turn into something more sinister as Wagner sequences it down and down and down, in a wonderful expository passage (unfortunately often cut, as it is in Seattle Opera’s current production, because of the inhuman demands it makes of the performers) known as the “Day and Night” discussion. Here Tristan explains how Day forced him to return to Ireland to win Isolde for Marke:
Was mir so rühmlich schien und hehr,
das rühmt' ich hell vor allem Heer;
vor allem Volke pries ich laut
der Erde schönste Königsbraut.
Dem Neid, den mir der Tag erweckt';
dem Eifer, den mein Glücke schreckt';
der Mißgunst, die mir Ehren
und Ruhm begann zu schweren:
denen bot ich Trotz,
und treu beschloß,
um Ehr' und Ruhm zu wahren,
nach Irland ich zu fahren.
You seemed to me so noble,
so worthy of fame,
I praised you loudly before all Cornwall.
I sang the praises of the earth’s
fairest bride before all Marke’s people.
Day aroused envy of me;
my good fortune frightened people into jealousy.
Ill-will began to burden my own honor.
In order to defy it and protect honor and fame,
I promised to go to Ireland
and win you for Marke.
The excited violins and cheering trumpets of that passage are even more intense, a musical portrait of a wild crowd cheering a celebrity, when Isolde uses the Day motif to describe the same episode of the story, in her Act One narrative:
In Act Three, however, Day’s hateful, burning, scorching glare becomes a deadly all-seeing eye which Tristan must escape, fleeing to a world where there is no light, no sight, only sound: “Do I hear the light?”