The servers will come down for a restart on June 6, at 9:00 AM EDT / 13:00 UTC / 15:00 CEST (check this link to see when this is in your timezone). The downtime is estimated to take 2 hours.
The 10th Anniversary Event will also conclude at this time. Thank you all for joining together for this occasion, heroes! Normal daily login rewards will resume after the downtime.