Closing day is when you sign a slew of papers and the property is transferred. Make sure you have all the documents you need to take to closing, conduct a preclosing walkthrough of the house, and do stretching exercises for the muscles in the wrist and hand of your writing arm. You're going to sign your name to more legal documents and checks than you can imagine, so limber up.
Pay especially close attention to the Closing Disclosure, which details the final closing costs. Compare it to the loan estimate and ask about any discrepancies.
You almost certainly will have an escrow account, from which the mortgage servicer pays taxes, property insurance, and maybe mortgage insurance. A portion of each monthly mortgage payment will go into escrow. At closing you will have to deposit some money into escrow as well.