No month is better for the score itself. GMAT Focus isn't graded against whoever happened to test that day, so there's no soft month to hunt for. What the calendar actually changes is how much room you leave yourself.
Work backward from the round you're targeting. R1 for this cycle is closing right now (Wharton and Ross on 8 September, Kellogg and Columbia on the 9th, Booth and Yale on the 15th), so you're realistically looking at R2, which clusters in the first two weeks of January. Wharton, Columbia and HBS are on the 5th, Kellogg and Stanford the 6th, Booth the 7th, MIT the 12th. Back that up and you want to be finished testing by mid-November. That covers the 16-day minimum before a retake, gives you a second sitting in early December if the first one goes sideways, and still leaves a clean month for essays.
One thing I'd add from self-studying around a full-time job: pick the test date first and work backward, because an open-ended timeline just stretches. The gap that matters is the one between your last sitting and your first essay draft.