OnInitialize is called in response to the next ADDON_LOADED or PLAYER_LOGIN event that fires. In most cases, this will be the ADDON_LOADED event for your addon.
OnEnable is
also called in response to the next ADDON_LOADED or PLAYER_LOGIN event that fires, but only if you are already "logged in" -- eg. the event
is PLAYER_LOGIN,
or PLAYER_LOGIN has already fired and your addon was loaded on demand afterwards -- and all addons have already had been initialized. In most cases, this means your OnEnable will get called when PLAYER_LOGIN fires.
AceAddon is actually quite well documented with comments, and the code is fairly easy to follow, so I'd encourage you to take a look through if you plan to use it much. It doesn't even register PLAYER_ENTERING_WORLD.
