How Do I Read Chat History On Facebook?

It varies everytime yo log off... Meaning you cant read chats from different sessions that you were on earlier that day

Facebook does not enable its users to view chat history.  
But according to facebook:
"You can view recent conversation history by opening a chat window with
your friend. You cannot view older conversations at this time or
conversations with friends who are not currently online. To clear your chat history just click the 'Clear Chat History' link at
the top of your Chat window. While message history is saved from page
to page, and even between login sessions, please note that it is not
logged permanently."Personally, I prefer to use Digsby to chat with my facebook friends. Digsby is a free downloadable multiprotocol IM client that lets you
chat with all your friends on AIM, MSN, Yahoo, ICQ, Google Talk,
Jabber, and Facebook with one simple to manage buddy list. Digsby has many other cool features which you can read here: www.digsby.com

FYI, you can log chat history in digsby and search for any chat log through the search-enabled log viewer.