Domains are released after the end of their term if they have not been renewed. Domains that expire at another registrar are subject to their own policies; however, there is usually a grace period, and then a period when the domain is “on hold”- and cannot be registered. You should allow at least 30 days from the time of expiration before you can try registering the domain.
To find out a registrar’s specific release policy, you should contact the registrar that owns the name. You can find out who is the registrar for a specific name by entering the name into the central database search engine at http://www.internic.net/whois.html.