Sounds like one of the scheduled maintenances. You might find these links helpful:
1. Tips on graceful degradation for your app (Java specific): http://code.google.com/appengine/docs/java/howto/maintenance...
2. Scheduled Maintenance Calendar: https://www.google.com/calendar/b/0/embed?src=4chlpo92ci2f54...
3. Downtime Notification Group / Mailing List: https://groups.google.com/forum/?fromgroups#!forum/google-ap...