If the closing date is not met by the buyer, what rights does the seller have?
You can sue them for any costs or losses this causes you, such as the additional carrying costs for holding on to the home longer, if not otherwise paid (e.g. at closing or otherwise by the buyer).
After several delays (as your question implies there already have been), you can send the buyer a "time is of the essence letter" in which you delieate the reasons you need to consummate the sale (e.g. you need the money to purchase other property, or you can't afford to pay for two places--the one you are selling and the new place you have/are moving to) and you also reiterate the latest agreed-upon closing date; the letter can go on to state that if they do not close then, you will consider them in breach of contract, terminate the sale and retain their deposit as damages or compensation. The letter should also restate all the previous missed closing dates. You have to give them a last warning (which you have to make sure they get prior to the date--e.g. email, fax, and/or overnight with tracking), but having given them a warning, if there have already been multiple missed closings, you can then consider them in breach.