My suggestions:
I would say have it on a Friday night (say 8 pm Eastern or later), or a Sat any time after, say, 3 pm Eastern. Those are generally near the time of FSO's and Scenarios that are calculated to fit the most player schedules.
I would probably (given we are close to weekend already) not pick to have tomorrow or this weekend.
I would pick a time and choose a week from now, giving people time to know for sure the day and time and be able to make time for it in advance.
If people haven't given feedback, assume they don't have a strong feeling on it. Pick from among the above choices, and it will tend to be a decent choice.
I can set up the arena for you to be the terrain you want, can include clouds or not as you see fit, and set time of day for a sunrise, sunset, noon, or whatever you want.