Oct. 1 is the deadline to get your Annual Publisher's Statement to your local post office.
It doesn't have to be printed in your newspaper by then, but the report should be in the hands of your postmaster or postmistress on or before Oct. 1.
A PDF of the newest statement can be downloaded here. It was updated last year to include an opportunity to claim online subscriptions toward your total circulation numbers.
The form or the information contained in it must be published in your newspaper. Basically, those printing more frequently than weekly must print the Annual Publisher's Statement by Oct. 10 and those printing weekly or monthly must print it before Oct. 31.