What's he saying?
"How can I then return in happy plight, / That am debarred the benefit of rest?"
How can I continue on when I cannot sleep?
"When day's oppression is not eas'd by night, / But day by night and night by day oppress'd,"
I am overcome with thoughts of you day and night,
"And each, though enemies to either's reign, / Do in consent shake hands to torture me,"
It is as if day and night, though usually at odds with each other, have made an agreement to torture me.
"The one by toil, the other to complain / How far I toil, still farther off from thee."
Day tortures me with work, and night tortures me with the realization that no matter hard I work, I seem to be getting farther away from you.
"I tell the day, to please him thou art bright, / And dost him grace when clouds do blot the heaven:"
I tell the day that you are bright like the sun, so that the day can be bright even when it is cloudy out:
"So flatter I the swart-complexion'd night, / When sparkling stars twire not thou gild'st the even."
And I tell the dark night that you are the bright stars that make it light.
"But day doth daily draw my sorrows longer, / And night doth nightly make grief's length seem stronger."
But every day seems longer than the last, and every night makes me suffer more than the last.
Why is he saying it?
Sonnet 28 is a continuance of Sonnet 27, which declared that the speaker cannot sleep because he is kept awake with thoughts of his love, who is far away. Now, the poet complains that he can find no rest during the day, when he must "toil," nor at night, when he is kept awake with the idea that no matter how hard he works, he is still far from his love. The first lines ask, "How can I then return in happy plight / That am debarred the benefit of rest?" The "return" is from the nightly journey made by his thoughts, described in Sonnet 27.
The speaker personifies Day and Night as forces that, though mutually exclusive and usually at odds with each other, are working together to "oppress" him. They "shake hands" as if they are two businessmen completing a transaction. Usually, "day's oppression" would be "eas'd by night," in that the speaker could get some rest after a long day of traveling; but he complains that this is not the case. It seems to him that Day is oppressing him by "toil," or by the physical journey he is undertaking, while Night forces him to consider how far away he is from his love.
Lines 9-10 describe the poet's imagined bargaining with Day. He hopes to convince Day to stop oppressing him by flattering it: "I tell the day, to please him thou art bright, / And dost him grace when clouds do blot the heaven." "Thou" refers to the fair lord; the speaker tells Day that his beloved is "bright," so that even when it is cloudy, Day can be just as beautiful.
Likewise, the poet argues with Night in lines 11-12: "So flatter I the swart-complexion'd night, / When sparkling stars twire not thou gild'st the even." "Swart-complexion'd" means dark, or swarthy, as the night's face would be in this scenario. Again, "thou" is in reference to the beloved of the poet, who shines to make the night beautiful even when the stars "twire not," or do not twinkle. "Gild'st" means to make gold, and "even" refers to the evening. So the lover serves as a substitute for the sun during an overcast day, as well as for "sparkling stars" during a cloudy night when they do not twinkle.
Unlike other sonnets, Sonnet 28 does not end on a hopeful note. Often, the final couplet can serve to change the direction or tone of the preceding poem, but in this case, the couplet only sinks the poet further into despair. The argument that the poet had with Day and Night in the previous lines was fanciful, so it is no surprise that it proves useless: still they torment him. The use of the word "draw" implies that Day is a torturer using either the rack, a mechanism that stretched the body of the victim, or executing the method of "drawing and quartering," in which the victim's entrails were removed and the body was cut into four pieces.