The current schedule lists the fortress siege vulnerability times in Aion.

Fortress siege times in Aion are no longer random, so NCsoft has provided North American players with a copy of the current siege schedule. The Assault on Balaurea expansion added four new fortresses to the game, and their addition into the current schedule is an ongoing process. Don't be surprised if some changes are made to the schedule in the future.

You can view the current siege schedule on Flickr in case you need it for future reference. Which day is your busiest day in Aion?