Patch Day occurs when a new patch is applied to the live game from Blizzard's servers and applied to your client to continue to play World of Warcraft. They are, unlike hotfixes, scheduled to be usually on Tuesdays (weekly maintenance downtime) in North America and on Wednesdays in Europe.
- If additional problems with hardware or software are encountered during patch day, some or all servers may be down for longer than expected. This was a problem especially with the pre-expansion patches for The Burning Crusade and Wrath of the Lich King. This has occurred to a smaller extent with patch 4.0.1, but with the increased use of Public Test Realms the downtime was extended only an additional 3 hours after the originally proposed time.
- Major patches often attract large numbers of people to attempt to log in at the same time. This can cause wait times to log in, server instability, and other problems with game play including battleground and instance crashes. Again, these issues have caused little interference as of late due to the increased testing of patches on the Public Test Realms.
- Major patches have been known to "break" UI mods when they make changes to the API. For this reason, AddOn sites such as Curse and WoWInterface are known to have technical difficulties when masses of people are trying to upload their Addons. It is advised to check for updates a few days before patches are launched. But with increased usage of Public Test Realms by AddOn Developers many larger and more popular addons have suffered little when the new patch is released because they have been developing new versions with the PTR
- It is a good idea to visit the forums on patch day, especially the technical support forums if you are experiencing any technical difficulties or bugs. There will be a large number viewing the forums and posting but it is a good idea to post relevant information so that if there is a problem it will be more likely to be fixed in the next Hotfix.