This is hands down the most famous of Shakespeare's sonnets. The speaker compares the subject to a summer's day and points out the many trivial imperfections that a summer day might have, while implying that the sonnet's subject is perfect in those regards. The speaker insists that summer days are often too hot, and the season itself is too short but that the subject's manner is temperate and their beauty eternal.
The theme is one of immortal beauty. The speaker has encouraged the subject in past sonnets to have children. In sonnet 17, he states that, "were some child of yours alive at that time, you should live twice." In this sonnet, it is the first time that the speaker does not make mention of the subject's lineage. The speaker realizes that as long as people have "eyes to see," the subject's beauty will live on through the sonnet itself. Seeing as we're discussing it hundreds of years later, it seems the speaker was right!