Yes, you should be able to buy the domain when it expires, unless someone beats you do it or the current owner renews it.
There are companies out there that specialise in buying up expired domains and using them for generating ad revenue (because an expired domain will still have incoming traffic from other sites). Some also re-sell them for a significant markup (especially if the domain had large amounts of traffic before expiring). This is a form of domain parking.
Some domain registrars (like GoDaddy) offer the ability to backorder a domain, which involves paying up front for a chance at getting the domain immediately when it expires. Of course it still doesn't help if the current owner chooses to renew, and if multiple people backorder the same domain, it usually ends up on an auction.
Bottom line - just because a domain name is set to expire at a given date doesn't mean it'll actually be available for purchase at that time.