So I got invited to the Beta but it says my trial starts Friday evening. It lets me download the .exe, but can I download the game whenever or will I not even be able to download it until my session starts?

You can download it now, you just won't be able to play until they switch the servers on.

1. This is not the forum for beta stuff, ask it in the actual beta forums. The link is found at the bottom of the email you got

2. you can download and install it, just cannot play it.

You could always try and start the exe and see if it starts downloading rather than asking on the wrong forums first. There hypothetically should be a link to the beta forums in your email, you should ask all your beta related issues on that forum, and not the public one. It's against NDA to say anything more than you got into beta (outside of the beta forums), and make sure you're posting in the beta forums next time.

Also welcome to the non-beta forums, here's a fishy stick! http://www.imperial-library.info/sites/default/files/dogate_til_fishystick.jpg

These forums are not for discussing the beta, even problems like this.

Use the link to the beta forums that is in the email.