An application for the H-2B can be submitted any time, as long as the H-2B cap has not been reached already for that half of the fiscal year.
Usually, applications for the first half of the fiscal year are submitted on October 1.The USCIS receives petitions until the cap of 33,000 has been reached. Once that number has been reached, the USCIS will take no more applications.
Petitions submitted after this will be rejected, but the submission fees will be refunded and the rejection of the petition will not be recorded as a visa refusal. A random selection process is used to pick out the petitions to be reviewed. The random selection process will not be used however, if the number of petitions did not exceed the H-2B cap. The same procedure is carried out in the second half of the fiscal year.
Therefore, it is advisable to submit your petition as early as possible (preferably on October 1 or April 1) to increase your chances of having it processed and approved.