You can take it whenever.

When you log into your LSAC account on lsat.org, click "Writing Sample" at the top of the page. If you scroll all the way down, you'll find the section labeled "LSAT Writing Status and Launch" and that's where you can take the writing component. You can also familiarize yourself with the security check in as well as the format of the sample using their "Get Acquainted With LSAT Writing" tool as well! Hope that helps!

@smhnunn216 when I received the email from the LSAC saying that registration was open, I was given a temporary pw to use. Hope that helps!