I heard this poem on The Writer's Almanac and knew it was just right for this time of year. 

It's by Jonathan Galassi and appears in his book Left-Handed (Alfred A. Knopf, 2012). 

The last swim of summer 

ought to be swum

without knowing it,

afternoon lost to

re-finding the rock

you can stand on

way out past the

raft, the flat one

that lines up four-

square with the door

of the boathouse.

Freestyle and back-

stroke and hours on

the dock nattering

on while the low sun

keeps setting fin-

gers and toes getting

number and number ...

how could we know

we were swimming the

last swim of summer?