Submission season

I would think that Christmas break is a far worse time to query agents as the large majority are on vacation themselves. In my opinion, if the agents you're querying don't specifically say "we're closed to submissions in the summer" then go ahead and query them. Good luck finding an agent!

I don't think it affects your chances. Just, as you say, may lengthen the response time. July and August are huge vacation months for everyone, including editors and agents (my own agent is going on vacation for two weeks at the end of the month).

The only reason I'd caveat against submitting anything until September is that your submission will be part of a rather large piling of work waiting for the agent when they return.

Depending on who you talk to, there is NEVER a good time to query an agent or an editor. Can't do it in December 'cause of the holidays. Can't do it in the summer 'cause of vacations. Can't do it in the spring or fall 'cause of expos and conferences.

I've long come to the conclusion that it's all BS--send it out when it's ready. Not every agent/editor does everything and I'll never know what their schedule is anyway, so there's no point in waiting. If they're gone, the submission will be there waiting for them. If they're there, then it gets looked at a little faster. Either which way works--it's out the door, which is my goal.