So, all you aspiring authors, don't submit your manuscript to publishers, and don't send your queries to agents in the month of August. (To be fair, mid-November to mid-January is also a bad time, but for different reasons.)
If your manuscript is ready to go, you have several choices of what to do during the dead month. You can head to the beach, you can write your next novel ...
You can do research!
August is the ideal time to gather publishing ammunition. This is how to do it:
1) Make a list of agents. To assemble your list of ideal agents, go to agentquery.com and do a search on your genre (i.e. the genre of the book you want to sell). Make a list of all agents who are accepting queries in that genre. Then, once your list is assembled, go to each and every agency's website to make sure the information you have garnered from agentquery is current. While you are there, read the website, and study the bios of the agents. Do you think the agent will be a good fit for you? If not, take the agent off your list. Next type the agent's name into a google search. Look for entries from "absolutewrite watercooler" and from "querytracker." Read what other writers say about these agents. If writers report bad treatment, then cross the agent off your list. (None of the agents on agentquery will charge a fee, but some who don't may recommend editing services for which they receive a kickback. Don't query an agent who also offers editing services.) Also check preditors&editors for complaints. It will take you at least a month to assemble your list, so have fun.
Sci-fi and fantasy writers, go here for a list of agents: Agents Looking for Science Fiction and Fantasy Writers
Paranormal romance writers, go here for a list of agents: Agents Seeking Paranormal Romance Writers
2) Make a list of reviewers. The time to contact reviewers is three to four months in advance of publication. So, if you have finished your manuscript and plan to self-publish as soon as the last word is written, STOP. Don't publish until you have reviewers lined up, and all your publicity in place. (Interviews, ads, etc.) Go to List of Online Reviewers Who Accept Self-Published Books for a list of 170 book bloggers who review self-published books. Also go to Top 12 Sites for Finding Reviewers for links to several hundred more reviewers and review sites. Cull through these for reviewers who write reviews for your genre. Make a very long list. (You will need several hundred. Returns on review requests are modest.) Be sure to include their email address, or other contact information on your list.
3) If you have written short stories or poems, make a list of literary journals. Trust me, even those few journal editors who say they accept submissions all year round don't read in August. The best way to submit to literary journals is to have a list of 20 or 30 at the ready. If you are going for fame, then start your list with the journals that don't accept simultaneous submissions. If you simply want to get some writing credits under your belt, then make a list of those that do accept simultaneous submissions - and submit to all of them at once. In September. Go here to find short story markets: Top 5 Online Resources for Short Story Markets. Go here for poetry: Top 5 Resources for Publishing Poetry
4) Write templates of query letters and review requests. All letters, whether they are to an agent or a reviewer, need to be individualized. Always include the name of the agent or reviewer in your salutation, say how or where you found them, and tailor your initial pitch to that person's stated interests. That being said, there are some things that will not change from one submission to the next. Your blurb (for reviewers) or pitch (for agents) will be the same, as will your word count, genre, bio and other pertinent information. It is a lot easier to write a template query or review request and tweak it than to compose a new email for each submission. Once you are happy with your template, copy it and paste it into an email. Don't send. Just save it as a draft. Having a draft online will save a lot of time later.
Note: Writer's Digest has a great series on successful queries, which is one of the best resources you will find on how to write a winning query letter.