Deadline to quit before busy season

I work for a smaller CPA firm comprised of about 15 auditors. I want to accept an offer for another position but am worried that it may be too close to busy season right now to quit. I can see quitting at a bigger firm in September isn't that huge of a deal but since I work for such a small company I am concerned that the firm will be very upset with me. What do you guys think is a proper cutoff date to leave a firm? Do you agree that I am too late and should wait until next summer? Thanks in advance.