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Maintenance periods are common in MMOs. They are scheduled windows of time during which the servers become inaccessible (thus the game unplayable). This is to allow for various upkeep tasks to be performed.
Phantasy Star Universe's maintenance period is scheduled for a 5-hour time slot, once a week. However, most maintenances to date have only taken around 1 hour, and not needed the full 5 scheduled, resulting in the servers going back up sooner than expected.
Updates can occur during maintenance. However, not all maintenance periods result in updates. Currently, it is not often that there is a maintenance period if there are no scheduled updates. It will usually be cancelled for the week.
The following chart details when maintenance will take place in the most notable timezones.
Gameplay | Release | Requirements | FAQ | Differences | Story | Story characters | Scripts | Organizations | Glossary | Alphabet | Merchandise | Importing | Events | Updates | Maintenance | Translation errors