Summer ends this weekend. To mourn its passing, the publishing industry will be shut down for the week.
The literary agents, having no one to submit to, will be off to their summer retreat in the Hamptons. Your query will sit in their in-box, unread and possibly forgotten when they come back from their vacation.
Hungry agents are still around, as would be expected, reading queries and asking for manuscripts that they have plenty of time to read because there's nothing else for doing in the publishing business.
Don't stop querying, just because it's the annual summer break. Find some younger agents on your list. Send your letter to the rookies just starting out, or those who have recently set up their own shop.
Query the agents who only accept snail mail. Even if they're out of the office until the second Monday in September, your query will be in their office, physically and in tangible form, just waiting for the return of the agents to New York City.