I would assume that if you pre-purchase on or before the 25th you will be able to play the 25th, but if you were to pre-purchase on the 26th you'd get to start the 26th. Purely assumption though.
This is all their FAQ states about it: Headstart Access will begin for those who pre-purchased Guild Wars 2 three days before release, on August 25, 2012. Headstart Access will begin on August 27, 2012, for those who pre-ordered the game. (https://www.guildwars2.com/en/services/faq/).