When Kitty posts about these events she grabs the info from the JP wiki. So everything that she writes is mainly in terms of what JP version did. Which most likely will happen for us. In our case, however, our server resets at 0:00 PDT, not 05:00 Japanese time. A lot of the notifications that talk about reset hadn't been changed from that, however they did recently update the caves to reset at midnight instead of 5:00 am PDT.

So every night at Midnight PDT The Union Event resets (Demons go back to level 1, Buffs get removed, daily missions get reset, etc) and the highest numbers get recorded for rankings and then gets reset as well. So between the hours of 0:00-23:59 PDT you can hit as much as you possibly can and it will count towards the ranking for that day. At the end of that day, if you have done more than any of the days before, your ranking numbers will get updated. If you did not to better than a previous day then your numbers will remain the same. Overall rankings will then be adjusted based on who has done better than your best day.

If you go to the rewards section of the rankings it will show you your exact rank in the upper right corner. Then you can figure out what rewards you will be eligible for.