"Poem in October" begins with these solemn and joyful words:
It was my thirtieth year to heaven
That's a pretty dramatic way to say "it was my thirtieth birthday." Every birthday, this line suggests, is just a milestone on the way to death—and who knows how many more years it will take to get there?
To this speaker, the idea is an uplifting one. He's not just ticking off the days until he's dead. He's a pilgrim on the road to heaven. His sense of life as a mysterious, sublime, and thrilling pilgrimage will animate this whole poem: he'll spend the morning of his thirtieth birthday on a walk that mirrors that lifelong heavenward journey.
Even at the moment he wakes up, the speaker feels there's something special in the air this morning. Even as he lies in bed, he can see the landscape around his home in his mind's eye: his "hearing" of the "harbour," the "neighbour wood," and the "mussel pooled" and "heron / Priested shore" lays those landmarks out for him (and for the reader!) as vividly as if he were a bird flying above the whole scene. (Not coincidentally, the speaker seems to be describing Dylan Thomas's native Wales; this speaker has more than a little in common with his author.)
The speaker's instant familiarity with the landscape already suggests that he loves this place—and his language suggests he sees it as sacred. When he describes the shore as "heron priested," he's not only evoking the stern, stiff-legged, priestly dignity of herons (though he's certainly doing that). He's also hinting that nature is innately holy. The "shore" here becomes the herons' church; just by living their lives, these birds preach a wordless sermon.
The speaker will make his journey into this landscape in seven stanzas of free verse. While the poem won't use any regular rhyme scheme or meter, it will still take on a regular, pulsing shape: each stanza uses a predictable pattern of longer and shorter lines, evoking both the hilly landscape the speaker will soon climb into and the larger rhythm of life he'll explore.