For some reason the developers refuse to allow this, and last I checked the form fields were not of the proper type for some apps to remember the username/pass on your behalf. Here is what I have done:
1) Create a new text file anywhere on your computer, like in c:\FILES. Rename it to fitday.html
2) Paste exactly this into that file:
<form id="form-login" method="POST" action="https://www.fitday.com/fitness/Login.html">
<p><label for="LoginName">Username</label><input id="username" type="text" name="LoginName" value="PUTYOURUSERNAMEHERE" /></p>
<p><label for="Password">Password</label><input type="password" autocomplete="off" name="Password" value="PUTYOURPASSWORDHERE" /></p>
<p><label for="login"></label><input type="submit" name="login" value="Login" /></p>
Be sure to replace PUTYOURUSERNAMEHERE with your username and PUTYOURPASSWORDHERE with your password.
This file recreates the functionality of the fitday.com login page, and submits your username/pass to the same page the normal login page does. The page AFTER the regular fitday.com login doesn't care who submitted the username/pass, just that they are submitted, so in this case you're submitting it from your local machine. This won't affect any functionality whatsoever once you're logged in to fit day.