Name changes are open every 6months give or take, although they need to be manually opened, so occasionally they'll be a little bit late.
Generally this means twice annually, though it might run into one season before or after.
New users can change their names as much as they want upon first joining. On each section that you haven't posted in, going to the user section of your profile will allow you to change your username. After posting once in each section, you'll lose this ability, so chose your name quickly.