Yaroslav, there hasn't been to much discussion about this topic, so I went ahead and suggested my plan to you ;-)) But obviously it would be nice for the countries to decide by themselves when they want to turn on the UploadCampaigns and CentralNotice banners (though it will have to be communicated to us beforehand, as there aren't so many active Meta admins here).
I would disagree about having the same time for starting and closing the contest. Our goal is to have as many pictures as possible -- in order to get as many new editors as possible -- and human beings have this strange habit of doing things at the latest possible moment. We've seen it last year, when -- as Romaine wrote in another thread recently -- about 25% of the total number of uploads came in on the last day.
It would be a great loss if we disabled the campaign on, say, October 1 00:00 Eastern Daylight Time (this is the timezone for the US states of Massachusetts, New York, Pennsylvania, Virginia, and the Canadian provinces of Ontario and Quebec), when there could be plenty of people from the other US timezones wanting to upload pictures /at the latest possible time/.
Adding some three or so hours for people to use in order to be "fair" to the other countries is not really a big deal, but there is potentially much to loose (in number of pictures that wouldn't make it to the contest).