Is there any predetermined time when full servers can potentially be opened or it happens randomly?

Is there any predetermined time when full servers can potentially be opened or it happens randomly?

it happens when wvw population of that server is to low over a longer period of time of weeks mate.

Server population is nothing to do with the number of people currently online, instead it's determined by the number of people who the game considers to be active WvW players (not in WvW right now, just playing it regularly). So it won't change quickly and there isn't any time of day when it's more likely to drop.

I think it's updated at least once a week, maybe more, but an individual server's population won't necessarily change with every update. The good news however is when it does change it will also be a long time before it changes back, so you don't have to worry about missing an opportunity to transfer unless you're away for weeks.

(Also this is a relatively short-term problem. We don't know when it will happen yet, but Anet are planning to drop servers for WvW and switch to a system of alliances determined by your guild, so joining up with the people you want to play with will become easier.)