CK9 - Any day can turn out badly in terms of people coming or not. And lets face it, it's hard to know your schedule a long way upfront.
However, I would chose dates so that they avoid certain situations during the year, like exam periods, school/university starting, major holidays (Exception here might be Christmas which in my part of the world involves going home to family to relax).
Other than this.. it's just a matter of trying. We used to have events for both summer and Christmas a few years back.. both turned out ok.