"Q: When is Tenth Edition legal for Constructed?
Anthony, Somerville, MA, USA
A: From Scott Larabee, DCI Program Manager:
Tenth Edition becomes legal for all Constructed formats on July 20, 2007. In Standard, Ninth Edition will no longer be legal when Tenth Edition becomes legal. Please note that if your National Championship starts on or after July 20, Tenth Edition (not Ninth) will be used for your Nationals for the Standard format portion of the event"