Most aircraft end up returning to Nellis AFB, so an amazing assortment of hardware can be seen on the ramps there most any time during a Flag. It's hard to say just when during a Flag things get hot, but here are some tips. I'm told that after the Gulf War, night training increased considerably, as this seems to be the way we'll fight in the future. So the after dark entertainment is often hopping, even though it's sometime difficult to tell what's going on. That doesn't mean it's quite during the day by any means.
Weekends can also be quite good, but there is probably a little more activity on weekdays.
The periods right at the start and end of each two week interval are likely going to be quiet, due to crew rotation, so you might want to avoid those.