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Funny, I've been wondering the same thing (low pop, JP server). For our EN community, our Wednesday evenings are slammed; by the weekend it's pretty damn quiet (except Sunday nights). My guess, like mentioned upstream, is that folks have more time on weekends and choose to get out of the house for a while. On weekdays, there isn't as much time or energy to go anywhere so they spend their free time in game.
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JP server I think also dies entirely on the weekend. I've never even once encountered a PF as full as work days on weekends, for the same reasons, I usually do stuff I can't during the weekdays like shopping, cooking.. spending time outside the game. If anything, people are hyped when lockouts are released so that's when it's the fullest.