Mon Jul 17, 2017 6:37 pm
The last few first flights have been announced a few weeks in advance, but those dates are always subject to change. The weather (particularly from October-May/June) can be unpredictable and certain adverse conditions (including low clouds/visibility) can postpone an early flight test flight. Additionally, if anything is caught during tests leading up to first flight, first flight could be postponed to address the issue.
Usually you'll see a date where the flight test "window" opens. This indicates where in the schedule flight testing can begin (ie, it won't happen earlier as pre-first-flight testing won't be complete), but other things can push the schedule to have first flight at a later date.