Sonnet
It was, however, Sir Philip Sidney's sequence "Astrophel and Stella" (1591) that started the English vogue for sonnet sequences. The next two decades saw sonnet sequences by William Shakespeare, Edmund Spenser, Michael Drayton, Samuel Daniel, Fulke Greville, William Drummond of Hawthornden, and many others. These sonnets were all essentially inspired by the Petrarchan tradition, and generally treat of the poet's love for some woman, with the exception of Shakespeare's sequence of 154 sonnets. The form is often named after Shakespeare, not because he was the first to write in this form but because he became its most famous practitioner. The form consists of fourteen lines structured as three quatrains and a couplet. The third quatrain generally introduces an unexpected sharp thematic or imagistic "turn", the volta. In Shakespeare's sonnets, however, the volta usually comes in the couplet, and usually summarizes the theme of the poem or introduces a fresh new look at the theme. With only a rare exception (for example, Shakespeare's Sonnet 145 in iambic tetrameter), the meter is iambic pentameter.
Sonnet 33
Although Sonnet 33 is considered a part of the group of Shakespearean sonnets addressed to a young man, there have been claims that the third quatrain of sonnet 33 may have been co-addressed to Shakespere's only son, Hamnet, who died in 1596 at the age of 11. As noted by Mark Schwartzberg of New York University, Shakespearean biographer Anthony Holden suggests that the death of Hamnet was "one of the catalysts that sparked the change to a darker tone in his [Shakespeare] work, a tone that increasingly reflected personal grief". This change in tone is found in "Hamlet", "The Winter's Tale", and "King John", which contained several, unexpected emotional moments.
According to Scwhartzberg, the sense of betrayal and disappointment that permeates throughout the poem from the speaker isn't necessarily directed at the fair, young youth that this sonnet is thought to be addressed to, but rather at God or Fate for taking the life of Hamnet too soon. Schwartzberg believes that there is a pun on the word "sun" which, when replaced with "son", provides the poem a tone of grievous loss. "Even so my sun one early morn did shine" (9) may be in reference to the brevity of Shakespeare's own son; "one early morn" being the phrase that captures this notion. This line combined with line 11 and 12 respectively, "But out alack, he was but one hour mine, / The region cloud hath mask'd him from me now" demonstrates Shakespeare's grief over the loss of his son as a father who felt like the existence of his son's life was but "one hour".
Michael Wood also suggests the allusion of the third quatrain of Sonnet 33 to the death of the poet's son with an implied pun on "sun". In "In Search of Shakespeare", he suggests that this sonnet might have nothing to do with the so-called "Fair Youth" sonnets, that it alludes to the death of the poet's son, Hamnet in 1596 at age 11, and that there is an implied pun on ""sun"" and ""son"": ""Even so my sun one early morn did shine, with all triumphant splendour on my brow; but out, alack, he was but one hour mine, the region cloud hath mask'd him from me now"". If this is the case the link of this sonnet with sonnets 34, 35 and 36 would be entirely coincidental and spurious.
Kerverne Smith believes that the emotional effect that Hamnet's death had on Shakespeare resulted in recurring features found in Shakespeare's later plays, which fit into one of five motifs: "the resurrected child or sibling, androgynous and twin-like figures, a growing emphasis on father-daughter relationships; paternal guilt; family division and reunion". If this notion is to be true, then sonnet 33 could be that of the "paternal guilt" motif. 
In other readings of the third quatrain, it should complete the metaphor of the young man as a sun, but fails to commit, "my sun" being a step removed from "my friend" or "my love". This requires the reader to allow the speaker some leeway toward the metaphor, not to search to deeply into the "nature of the region cloud or its masking". The third quatrain restates the first two quatrains in the same metaphoric terms.