“Those Winter Sundays” fills the most basic qualification for a sonnet: it has fourteen lines. Other than that, it’s not very sonnet-ish. The poem doesn’t rhyme and it’s not written in regular iambic pentameter.
We mean, sure, the poem begins with two ten-syllable lines:
Sundays too my father got up early
and put his clothes on in the blueblack cold,
but if you scan them (as we’ve done above), you realize that the first line has no metrical pattern, while the second line is in perfect iambic pentameter. And then if you keep reading, you discover that the third line isn’t even close to being ten syllables long and the rhythm is more than a little wonky:
then with cracked hands that ached
This line follows no metrical pattern whatsoever. It keeps changing its M.O. in every line.
At this point, we’re ready to wave goodbye to the land of regular meter. If you gave “Those Winter Sundays” to your pal Shakespeare (sonnet writer-extraordinaire), he might not recognize it as a sonnet at all. That’s how un-sonnet-y this sonnet is.
And yet. And yet! “Those Winter Sundays” is a poem about love. And what are traditional sonnets about? Did someone say love? Yup, that’s right: sonnets are usually about good ol' ooey gooey, hearts and flowers L-O-V-E love.
Even though Hayden’s poem isn’t about romantic love (as so many sonnets are) it is about deep and abiding love—the love that a father has for his children, and the love that children have for their parents. “Those Winter Sundays” isn’t filled with hugs and kisses, but that doesn’t make it any less a love poem.
We also think that the secret sonnet-ness of the poem’s form connects up with its content. Just as the speaker doesn’t understand the nature of his father’s love until he’s grown, we don’t understand the poem’s form to be a sonnet until we finish it (and realize in retrospect that it has fourteen lines). So there's a delayed, or belated, recognition of love (and its many forms) for both the speaker and for us, the readers.
Neat, isn’t it? Robert Hayden really knew what he was doing.