When is the seller required to deliver possession of the property to the buyer?

According to paragraph 13 of Form 2-T, possession of the property, including all means of access to the property (i.e., keys, security codes, etc.), must be delivered to the buyer upon Closing, as that term is defined in paragraph 1(m) of the contract, unless (a) the parties have agreed otherwise, or (b) possession is subject to the rights of one or more tenants.