The reason i suggested saturdays is because most people don't work during the weekends.
By that i mean.. if you do it on sundays.. people will have to work the next day. Wich will make it impossible for those who will have woe around midnight.
And why i suggested 6pm GMT is simply because that would be the best time globally that people could play during the weekend.
Morning in America, afternoon/evening in EU and not too deep at night for Asia