What's he saying?
"So am I as the rich, whose blessed key / Can bring him to his sweet up-locked treasure,"
I am like a rich person whose wonderful key can open up his dear, locked-up treasure,
"The which he will not every hour survey / For blunting the fine point of seldom pleasure."
Which he will not visit too often for fear of dulling the excitement of experiencing a rare pleasure.
"Therefore are feasts so solemn and so rare / Since, seldom coming, in the long year set,"
That is why feasts are so special and rare, for they occur so seldom throughout the year,
"Like stones of worth they thinly placed are / Or captain jewels in the carcanet."
Sparsely placed like precious stones, or like the largest gems in a jeweled necklace.
"So is the time that keeps you as my chest / Or as the wardrobe which the robe doth hide,"
Similarly, time (or memory) keeps you like my treasure chest, or like a wardrobe hides the robe within,
"To make some special instant special blest / By new unfolding his imprison'd pride."
Awaiting some special occasion to be brought out, to uncover the pride that has been imprisoned.
"Blessed are you, whose worthiness gives scope / Being had, to triumph, being lack'd, to hope."
You are blessed, you whose worthiness gives measure; to have had you is to triumph, to lack you is at least to hope.
Why is he saying it?
Sonnet 52 is wild, hotly contested among scholars for its (possible) abundance of sexual innuendo. It also can be argued that because sonnet 52 comes later in the sequence than sonnet 20, sonnet 52 represents a later stage or evolution of the poet's desires - but arguments based purely on the sonnets' ordering are shaky at best, since some scholars believe that the ordering of the sonnets does not conform to any actual chronology of events.
In sonnet 52 the poet describes the fair lord as a locked-up treasure, a solemn feast, a robe for a special occasion - something special and beautiful and blessed, as only something so rare can be. The language of the sonnet is overtly laudatory and also rationalizing, as it attempts to justify the narrator's separation from the fair lord or the infrequency of his being able to delight in him. As though only permissible on special occasions, the robe is awaiting its chance to come out of the closet, "To make some special instant special blest / By new unfolding his imprison'd pride."
Note the possible sexual innuendo captured in the seemingly phallic "fine point of seldom pleasure," the penetration of a key into a lock, and the "unfolding ... pride." Also note that the word "had" (line 14) is found elsewhere in the sonnets referring to sex, cf. "Past reason hunted, and no sooner had . . . Had, having, and in quest to have, extreme" (sonnet 129). Such are the clues that have led some scholars to the idea that sonnet 52 is in fact a revelation of the poet's having been sexually attracted to the fair lord.
But did Shakespeare really intend for this sonnet to be read as replete with sexual innuendo? Or is it just readers with a modern way of thinking who are taken aback by its amorous language and led to draw conclusions that are merely the products of our own imagination? These questions apply not only to sonnet 52 but also to the sonnets as a whole; however, in sonnet 52 the language seems to cross the line, warranting some attempt at explanation.
Some scholars have argued that the sonnet clearly expresses the narrator's homoerotic desire for the fair lord, while others suggest that if there were any such desire on the part of the poet he would have taken better care to hide it, as homosexuality was viewed as a serious crime during Shakespeare's time, and he could very well have been punished for it. Critics of this latter conviction sometimes propose the alternative interpretation that whatever innuendo present in sonnet 52 is there for the sake of humorous double entendre, while others deny its existence outright. As with many of the sonnets' enduring mysteries, Shakespeare's clever ambiguities are likely to remain as such forever.