http://www.swtor.com/preorder/faq#q209656 states that:
How will I be able to gain access to the Game in time for Early Game Access?
Customers who have redeemed their Pre-Order Code for Star Wars: The Old Republic will be notified via email prior to the beginning of Early Game Access that the Game client is ready for download. Another email will be sent at the beginning of Early Game Access to inform players that they can now access Star Wars: The Old Republic.
When does Early Game Access begin?
Early Game Access will begin prior to the official release of Star Wars: The Old Republic. Precise dates will be revealed at a later time.
Now, what else I have come across somewhere...IIRC, what I remember seeing/reading/hearing that early access would not be "just a weekend", but I also seem to remember someone saying not to expect 2 weeks.
Also, the earlier you ordered the game, the earlier you'd be able to sign in for early access.
Finally, we will be given access to download the client, and that's how we will be able to access the game come "Early Access" time.
Thank you!
Xijitok